Spontaneous decay of mass? Do all masses, small (quantum particles) or large (classical, stars) spontaneously (without an external cause) decay in time sooner or later?
 A: The standard answer would be "no". 
Electrons are as far as we know absolutely stable since charge is conserved and they are the lightest charged particle: there is nothing they could decay to without breaking charge conservation. Protons might not be stable since there are suspicions that baryon number is not conserved. So in the long run we should expect only the lightest particles with a conserved property to remain - neutrinos, photons, electrons/positrons etc. 
Now, there are some theories like Penrose's CCC that proposes that electrons are not absolutely stable, but they are pretty theoretical and have no empirical support. 
There is also a neat-sounding argument from John Baez that in expanding universes with finite temperature eventually all bound systems will become unbound since matter tends to minimise free energy $E-TS$. Normally that implies minimising the energy $E$ by staying in a bound state rather than maximising the entropy $S$ when the temperature $T$ is low. But at a finite nonzero temperature $T>0$ maximising the entropy $S$ becomes preferable if it can be increased more than $E/T$, which becomes increasingly feasible as space expands and there is more room for dispersal, allowing ever larger values of $S$. I do not know if this is generally accepted, but the argument suggests that everything but conserved particles would decay in the very long run. 
