Coral is porous and you may of just seen air bubbles coming off of it to be honest, you could put a snail in and see how it gets along?
A dedicated forum and online store for the Opae ula shrimp!
Can I put live moss balls in the jar for the opea ulas?
sure, they may not grow very fast if at all but they will sit there looking nice and provide a place for the shrimp to graze. Like with any plants though, if they start going bad.. throw them out. 
So is the moss like food for the opea or more just for decoration?