They only feed on micro algae, micro organisms, and detritus.
They spend most of their time under the sand, keeping your substrate from getting clogged and junk from building up on the surface, pretty beneficial creature. I'm surprised to see it on the glass. You might want to add some plankton of sorts to your tank. They don't tend to do well in new tanks since it's a pretty clean environment, but it might not have a problem if you feed the tank. I have one in the tank I keep at work. They have a reputation for not being very long lived in captivity unfortunately, due to lack of microfauna.
You might do some more research on it on other forums like Reefcentral and WetWebMedia.com.