It sounds like brown jelly disease, but I've only experienced it with some LPS in the past. I found it to be totally treatable by suctioning off of the brown jelly-like material, followed with a Lugol's dip, then keep the coral in a good alternating water flow. If the brown jelly reappears, repeat process.
Watch for reinfection at the edges of the receding edge of the coral.
Mitch
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