Black/Purple dots on montipora

[Dlealrious]

Hi guys. I was wondering if ayone knows what these black/purple spots are on my monti. It doesnt look like a bug as they haven't moved. Tried to brush off with toothbrush but they didnt come off. It's only started after i got a diamond goby and now have to blow sand off it daily

 

[Tahoe61]

Is algae, like coralline growing on exposed skeleton?

 

[Dlealrious]

Yup think your right, had a look this morning with lights off and it's sure looks like corraline very purple. I guess all i can do is hope coral grows back over it in guess