I did once have a couple of purple ones,they were also a pain in the butt. Everything they passed they collected.
In this rock I had a peacock mantis the pistol shrimp and a teddybear crab . However when I ordered it I told the chap those things didnt bother me....
The pistol shrimp has become a problem tho . Its also clouted a cleaner shrimp and killed it.
I didnt belive they could do this untill I saw a lovely little video on utube