Thanks for the replies. I think it's most likely hydroids, since I have just dosed the reef flux, which killed all the GHA and bryopsis in my tank.
Did a quick search on hydroids, they seem to be terrible pest.
@Jikkermanccini Do you have any experience with these critters? I did have a few Acros on that rock that died over night. Now I am suspecting that my Acros are stung by these hydroids.