What is this?

[hkgar]

It is about the size of a nickle and perfectly round. Only seem to have this one.

 

[hkgar]

Another view from a distance. Bottom of picture just right of center.

 

[jb rekit]

Looks like bubble algae. Don't pop it or the spores will spread it all over you tank.

 

[hkgar]

Good or bad? What to do with it?

 

[speg]

It's a bad thing. It'll end up going crazy in your tank if you dont take care of it soon. One of the best things for it - emerald mithrax crabs.

 

[jb rekit]

I can't tell from the pic, but if the rock it is on does not have a coral attached to it, you can take the rock out without popping the bubble and remove it in a bucket and rinse the rock off good before putting it back in your tank.
You can hold the rock upside down so if you do pop it, it doesn't get all over your rock.
and its not good because even though that 1 bubble might look neat, if you accendently pop it one day, you will have them alll over your tank and the problem will keep getting worse exponetially.

 

[hkgar]

Thanks all for the advice. I thought maybe the rock it was on was the rock my green star was on but no. It has been successfully removed.