ID help with this encrusting black stuff

[zedxplorer]

Hi all!

I need some help with this black encrusting stuff that is slowly spreading on my live rock. It came with a Christmas favia half a year ago. But since I have moved the favia, it seems to keep spreading. Anyone has idea what this is? Is it an algae, encrusting sponge, or something belong to the favia?

 

[KrisReef]

Can’t really tell from the pictures. Can’ it be dislodged with a blast from a powerhead, or baster?

 

[zedxplorer]

Not really, it's almost like mineral deposits, it also forms these circular discs shape at some area