Cyphastrea in trouble. Any ideas?

[bringbackplutonsp]

Hi there, I’m having some trouble with a master blaster cyphastrea in my 75g corner bowed tank.
My tank is about 7 months old, the cyph went in about 2 months ago. At first it was doing great, spread to the rock work super fast and I thought everything was great. Now it has this dark green/brown/black stuff on its right side. it looks like a tough mat growing between the polyps

In the second photo it’s just left of center. The top power heads aren’t on (it’s in a school and the students knock the lower ones off their magnets and sometimes it’s a pain to get them back on so I turn the top ones on in the meantime)

I haven’t moved it, but I did increase the white light a little bit in that time. I also added a cleanup crew (ceriths, nerites, astreas), which I think is the major issue as I had a zoa close up around the same time. All my other zoas and the other cyph seems ok, but I figured I’d ask here.

As for treatment, I tried a soft bristle brush and that did nothing.

Tank parameters
Salinity at 35ppt
Alk is 8.3
No3 is 10
Phos is 0.5
I have an autodoser with b-ionic, I do 20-30% waterchanges every two to three weeks.
I have a kessil a360we I keep mostly blue, and two Fluval strip lights that I have on white for a few hours around noon.

I’m open to any and all suggestions.

 

[Rtaylor]

Probably algae growing on dead parts of the coral. Cyphastrea tend to prefer low lighting. Try reducing the light or shading it with paper or something temporarily.