Aloha,
That's exactly what I was thinking. Commonly known simply as "yellow polyps". Sort of like the dreaded Majano they may spread quickly over rocks. Their powerful sting helps them expand their territory. Most often we see them in a drab yellow-brown color but every once in a while these bright yellow morphs show up, ...until they brown out again.
Curious where you acquired them and were there more
BTW, I would be careful not to let them attach to a rock you want to keep. Give them their own rock. I've got Utah Rock of all sizes. I'd be happy to give you a free piece for these to colonize.
Mahalo,
Mark
808-345-1049 call/text anytime to come visit and "talk reef"
Edited by Mark Peterson - June 11 2013 at 7:57am