Why is condensed milk glowing green under UV? Teeth, nails and also сгущёнка (condensed milk) are glowing green when exposed to black light. What do they have in common? Is it a specific luminophore substance?
 A: Short answer: autofluorescent proteins, most likely.
Many biological structures emit light after absorption of light at a different wavelength; this is called autofluorescence. "Auto" indicates that the fluorescence occurs naturally, i.e. without adding artificial sources of fluorescence. For example, melanin (pigment which determines skin color) absorbs light between 340-400 nm and emits between 360-560 nm while collagen (structural protein in bones, teeth, cartilage, skin, etc.) absorbs 270-370 nm and emits 305-450 nm.$^{1,2}$ As for the specific materials you list:
Teeth: under the hard inorganic enamel coating of teeth is the tissue called dentin and “The main organic matrix in dentin is the fibrous collagen [4], which generates intense second harmonic and autofluorescence (AF) signals [5–8].”$^3$ However, “The origins of autofluorescence from dental tissue may be quite diverse, and include both organic and inorganic components [4,29].”$^3$
Nails: they are mostly made of the structural protein keratin and “Strong keratin fluorescence with excitation and emission characteristics similar to collagen were observed in the topmost layer of the keratinized squamous epithelium.”$^4$
Milk: see below a figure from a study$^5$ which measured the autofluorescence of a number of proteins found in milk.
Please note that the autofluorescent proteins I've mentioned may not be the only sources of fluorescence in these materials. Check the references for more detail.

$^1$ James M. Gallas and Melvin Eisner (May 1987). "Fluorescence of Melanin-Dependence upon Excitation Wavelength and Concentration." Photochem. And Photobiol. 45 (5): 595–600. doi:10.1111/j.1751-1097.1987.tb07385.x
$^2$ 
Georgakoudi et al. (2002). "NAD(P)H and collagen as in vivo quantitative fluorescent biomarkers of epithelial precancerous changes." Cancer Res. 62 (3): 682–687. 
$^3$ Lin et al. "Imaging carious dental tissues with multiphoton fluorescence lifetime imaging microscopy." Biomedical Optics Express. 2011;2(1):149-158. doi:10.1364/BOE.2.000149. 
$^4$ Wu Y. and Qu J. Y. “Autofluorescence spectroscopy of epithelial tissues,” J. Biomed. Opt. 11(5), 054023 (2006).10.1117/1.2362741
$^5$ Gallier et al. "Using confocal laser scanning microscopy to probe the milk fat globule membrane and associated proteins." Journal of Agricultural and Food Chemistry, 58 (2010), pp. 4250–4257 
A: First to answer after 2 years.

Fluorescent substances absorb short-wavelength light (such as the ultraviolet radiation given off by a black light) and re-emit it almost instantaneously (usually in 10-6 to 10-9 seconds). The emitted light often has a longer wavelength than the radiation that was originally absorbed.
Not all substances can fluoresce. Fluorescent substances usually have highly rigid molecular structures with electrons that are delocalized over the entire framework of the molecule. Examples of common materials that contain fluorescent molecules are:
White paper is treated with fluorescent compounds to make it appear brighter. Historical documents that have been forged can sometimes be detected by placing them under a black light to see if they fluoresce; fluorescers have only been used in paper made after about 1950 1.
Club soda or tonic water contains quinine, a bitter-tasting fluorescent compound that glows blue-white under a black light.
Vitamins (specifically vitamin A, thiamine, niacin, and riboflavin) fluoresce strongly. A small piece of a vitamin B-12 tablet crushed and dissolved in vinegar glows a bright yellow under UV light and combined with the purple hue of the UV light could appear green.
Chlorophyll fluoresces with a blood red color. Grind spinach or swiss chard with ethanol and filter the resulting chlorophyll extract. Bright white light can be used to drive the fluorescence. Place the extract in a petri dish on top of an overhead projector lamp in a darkened room to get the full effect.
Many biological fluids contain fluorescent molecules. Forensic scientists use UV lights on crime scenes to detect fluorescence from blood, urine, or semen. Some marine organisms (notably jellyfish) produce proteins that are very intensely fluorescent.
Antifreeze fluid contains additives that fluoresce. Investigators sometimes use black lights to look for antifreeze splashes on pavement and vehicles to reconstruct automobile accident scenes [2].
Laundry detergents contain fluorescers to make the whites "whiter than white". Detergent residues on white clothing causes it to glow blue-white under a black light. These fluorescers sometimes make white clothing appear blue in color photographs.
Dental enamel and whiteners also contain fluorescent compounds that fill in the blue part of the spectrum to prevent the enamel from appearing too yellow.
Postage stamps are printed with inks that contain fluorescent dyes.
Some common minerals and gemstones fluoresce, including gypsum, talc, opal, agate, quartz, and amber. For a complete list see Amethyst Galleries' Fluorescent, Phosphorescent, Thermoluminescent and Triboluminescent Minerals page.

From Fred Senese's Frostburg General Chemistry online Q&A
