is this brown jelly disease?

sorrry yes it is a merulina

 

I've always kept Merulina up higher...IME they like a lot of light and have requirements more like most Acro's. Not sure that is any issue here though. I don't think Alk and Mag a little higher wouldn't hurt if those readings are accurate. I've found Merulina to be pretty tough...I've had them stung/damaged by other corals and the tissue always grew back well. Can't think of any other suggestions to offer.

 

~ Agreed. Looks ok, no new die off. Tissue look healthy

 

If you see any brown jelly ie. necrotic tissue, you can use airline tubing to siphon off the dead/necrotic tissue and giving the coral a bath with an Iodine solution like TM Pro Coral Cure or similar product is likely the best way to treat the problem as the iodine will act as an antiseptic and hopefully kill prevent the spread of the tissue necrosis.