Smooth talk

[H-C6179]

Morning, reefers

Can anyone tell me what this smooth stuff is? the dark patch. Rubs off easy, always comes back in the same place after a few days no where else on rock Cheers

 

[Dan_P]

Morning, reefers

Can anyone tell me what this smooth stuff is? the dark patch. Rubs off easy, always comes back in the same place after a few days no where else on rock Cheers

Could be cyanobacteria.

 

[MrGisonni]

Cyanobacteria