Bubble on an acan(?)

[Kehy]

First off, not sure this is actually an acan, though it was sold to me as one. The dude at the store said it'd been there for months without any interest in it, and I was the sucker that bought it. It was originally brown/dark ring and green center, but has brightened up substantially... with no growth.

Last few days, possibly 1-2 weeks though it seems to have developed a bubble under its skin, with seemingly rapid growth in the last 48 hours. We have had issues with the light, but it's been stable for about a week.

Any ideas what the bubble might be? I'll admit to never testing anything, but the tank has seemed fairly stable. If need be, I can do a ~40% water change (3 gallon tank), and I did do a small water change last night.

Ignore the algae, as per usual. I put it in this corner to protect it from me smacking it while cleaning, and it's done well with that.

 

[odstar]

Really hard to tell. To me it seems to be just a unusual tissue growth.

 

[Kehy]

And today the bubble is just gone? As if it had never been there. Huh.

 

[thamnasteroid]

That's not an acan, it's a brain coral in the family merulinidae.

 

[Kehy]

That's not an acan, it's a brain coral in the family merulinidae.

I did figure it wasn't an acan, but that's all I had to go on. Identification isn't my strong suit.

It's also changed quite dramatically in terms of color. This was July 22nd