You are right about that. However, sheet moss is more likely to grow mold and turn bad is very mosit and wet envoirments, especially if the wetness is constantly. At least, that's what I've understand when reading about the different mosses. I could be wrong.
On a side note, when talking about RETF, the digestion of substrate is quite uncommon and should not happen, since they are not a ferocious eaters as other frogs. A food bowl should also be used, if you ask me, and that effectivly reduces the impaction-risk.