Black bubble on zoas

[marioannou]

Hey, I noticed these two black bubbles on two polyps. The polyp with the smaller one is open and the one with the bigger is closed. Any ideas what they are?

 

[reef lover]

Hard to see. Might be bubble algae.

 

[Mikedawg]

Yeah, I would guess bubble algae altho I think it is usually dark green. Maybe remove the plug from tank and then scrape off bubbles to be safe.