I had a Frogspawn a year ago that had the same thing. It was doing great and one morning it was receeded and had a HUGE bubble just off the center. It expanded more during the course of the day but the bubble remained. I watched it in the evening right after I fed the tank and to my surprise I noticed when it caught some of the mysis in the water column I saw my darn blue legged hermit come flying out and climbing all over it to pick off what the poor thing had trapped. The hermit would literally pinch and pull it tissue and thats what I believe must have caused the problem. The hermit would also pick at my open brain corals and I immediately removed the hermit and just gave it back to my LFS. After two weeks the bubble decreased in size and finally dissapeared.
Thanks everyone. I guess I will know in a week or so. It has been really healthy and has grown like crazy over the last year, but I have never noticed this kind of thing before. It too has moderate flow and bright light.