Monti cap bleaching from the middle out

[d3rryc]

I left town for the weekend and came back to find a green monti cap with a big white patch in the middle of it. The bleached area in the middle is ringed by a light brown area that looks like the coral expelling its zooxanthellae. From this morning to lunch, it does look like the bleached area is expanding. I've got a second green cap and an orange cap in this tank that are both doing just fine. Any ideas what might be causing this?

Derry

Current params:
Salinity 1.026
Ammonia 0
Nitrite 0
Nitrate 0
Phosphate 0
pH 8.3
dKH 13
Ca 320
(I got a bad Ca test kit that was throwing my readings off for a while - need to bring dKH down and Ca up)