Fungus? Good? Bad?...Please ID

[Mouse51180]

I noticed this growing on a rock in the back of my tank recently. There seems to be more of it growing underneather the rock then on top. Any idea what this yellow looking fungus might be? and is it something to worry about?

Thanks

 

[Anonymous]

It's not fungus is a species of Porifera (sponge), harmless.

mario