Aaaargh Before long they'll be all over your tank, hiding in your rocks, stinging you corals and your fish, basically become a real pest.
I bought one rock with almost 20 of them a few months ago. Like the colour but didn't exactly know what it was. After placing it tank, they start to dislodge and move all over the tank You can't remove them as they will shrink into your rocks crevice. Beware.