Dark green spots on rock

[wd2424]

is this green coraline or something else? Doesn’t seem to brush off easily

 

[vetteguy53081]

is this green coraline or something else? Doesn’t seem to brush off easily

Looks like film algae but best determined under white lighting.

 

[wd2424]

Would film algae not brush off with a toothbrush? I’ll try and get a picture under white light