Bubbles on my zoa?

[OverkillNANO]

Last night I noticed this bubble under the skirt of this zoa. When I woke up today it was gone. The bubble was stiff enough that when I tried to poke it with my syringe it just bounced off it. I’ve attached a photo from last night. Any help is greatly appreciated

 

[Crossfire]

probably bubble algae. I've got some bubble algae growing on one of my green star polyp frags.

 

[OverkillNANO]

probably bubble algae. I've got some bubble algae growing on one of my green star polyp frags.

It was gone by morning and hasn’t returned . I’m at a loss for thoughts lol