Yours was an algae from what I got from the photo. I hope you took the opportunity to double check as the photo wasnt all that in focus IIRCI had something similar was type of algae. @TORX identified it for me.
ThanksThis is a typical yellow sponge. They are reef safe typically but some can be aggressive and over grow corals. It is easy to kill off, just take it out of the water. Air kills them off but be careful as it can cause an amonia spike. You can manually remove some while it is out of the water.
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