Yes I think it was a green star polyp but still. It come back out
+1 on the GSP
My green star polyp did that for about two weeks as I tried everything I could to get it to open back up. What worked for me was running carbon for a while and it opened back up in a day or two. From what I've heard they are sensitive to certain chems in the water and the carbon will remove them. Make sure your water is good and post your params. Good luck!