Sps receding from edges of base and black specs left on skeleton?

[BoomCorals]

If it is this... how is it treated?

AKA skeletal eroding band. No known treatment that I know of. Other than try to keep coral healthy. The bad news is they spread and can grow on almost any surface. You can manage it so long as the acros are healthy and tank is thriving.

 

[CoralExotic]

If it is this... how is it treated?

I'm dealing with this right now with one of my acros. Sadly, there's not much information on them so I'm just fragging the coral and re-gluing to a new plug. I don't know if they are causing the STN or just appearing after STN. Does your's look like this?