yes, they are safe... up to a point. If in sufficient numbers they will become opportunistic and eat some corals. They normally eat the biofilm algae growing on the glass and that is where you will see most of them. I had thousands, and they ate up my cheato ball. Got 2 harlequin shrimps and they took care of them within a relative short time span. So much that I had to sell the harlequins because they run out of food.
Think they also bother Monti and some SPS, if their numbers are too high. Others can confirm here, as I got a softy system. They did bother my Pipe organ coral which is a LPS. Just keep an eye out on them.