It's not poop. It's fly barf.
A fly spends about 25% of its time re-digesting and it only can eat liquids. It mixes the eaten food with the appropriate enzyme for digestion. The fly does this by retrieving the eaten food from its digestive system (a vomit of sorts), and drop by drop it is placed on the surface on which the fly is sitting. Only then is it sucked back up after they are mixed. The small black dots, that are left in various places, such as the ceiling, are not fly droppings, but actually the remains which are not sucked back up.
There are a variety of fluids that are mixed along with oils from the food. This creates a surface film much like when you write with your finger on a mirror. The growth of the spot is just the fluids slowly flattening out and spreading on the surface. The film prevents condensing water from beading to form a lens shape which defuses the light into the cloudy haze you see elsewhere. This is like an anti-fogging agent which works by minimizing surface tension, resulting in a non-scattering film of water instead of single droplets, an effect called wetting. Anti-fog treatments usually work either by application of a surfactant film, or by creating a hydrophilic surface.
The wetting may appear to spread near the edges of the film during evaporation because the droplets are on the cusp of being completely wetted and have more surface area to evaporate faster which gives the illusion of the spot spreading during evaporation.
As for the poop? Well, the droppings fall to the ground and go undetected.