I have literally thousands of these guys in my tank. They multiply like rabbits! I have never seen a perfectly shaped one either
My observations:
They eat detritus off the sand bed
They do a really good job of keeping the glass clean.
They keep my tank fairly clean as I have NO snails in it.
However, I was shown last week that in Sprungs new invertebrate book that they eat corals. Now I have never seen this, and the picture in the book is the same as the one you posted. So I don't know who to believe, but I have never seen one on any of my corals and I have about 15 SPS frags in my tank right now. Maybe Dr. Ron has some input.
Fade