I would say its brown jelly, it will kill the coral quick, I advise to Suck the jelly off of it with a Turkey baster/pipette or siphon off with air line tube, do this with the flow off so it has less chance of spreading. Then i would remove the coral as it's a goner I'm afraid. I lost a 10 year old wall hammer, it was big considering how slow they grow, but it was dead with in a week and I'm glad it didnt take my other hammers out.