What is this sphere on my rock

[Poli]

So my tank is well established not sure what this is any idea. Specifically I am talking about that little graying greenish sphere on my rock.

 

[Hollywood_XL]

Valonia (bubble algea) remove quickly

 

[Blake509]

Don't pop it in tank this will cause it to spread. If you do siphon out water around bubble as you pop. I usually avoid manual removal of bubble algae it usually makes it worse an emerald crab did wonders for me all but one little bubble was gone in 24hrs.