My montiporas were dying and had a brown jelly as well. I finally flipped them over and found Montipora Eating Nudibranchs. They are white and very tiny. If your other corals are okay, I would take a look for them.
Most of the time its nudibranches if everything else is doing good I don't its light burn these corals are pretty hardy. I had nudibranches problems before it took awhile to rid them you have to keep checking on the bottom cause they hide at the bottom usually.
