It's not all shrimp. Just certain ones. Mantis shrimp, and Camel back shrimp (if you have coral) are generally undesirable.
I bought three camel backs before I knew they were nibblers (although I "ASKED" the LFS dude if they were reef safe)......"Oh, yeah, sure they are".
I think two of them are gonners (mildly relieved), but there's still one left. I'm watching my coral like a hawk, and can't tell if there's anything going on or not. I have a lot of zoos, though, and "some" of them are looking pretty miffed about something.
I've also noticed one of the 3 of my emerald crabs getting pretty rambunctious and pulling on some of my zoo frag's polyps. He immediately met Mr. Algae scraper and was shood off.......much to his dismay.
I know if I put more food for them in there, they would be less likely to do that kind of stuff, but I also don't want my nitrates going through the roof.