Help with diagnosis and treatment

I have a 55 gallon freshwater aquarium that I have had set up for 3 years with 5 African Cichilids (all of which are the original fish). I have never had any problems with water quality and I keep my tank clean. The only thing that has been added to the tank recently (other than water), was a lava rock from the pet store to raise the PH. I just noticed that my yellow cichlid was vertical and sort of leaning against my castle in my tank a couple of days ago. I examined him more closely and he was covered in a sort of cob-web like white fuzz. I treated the water with stress coat and salt and did a water change. The next day he was dead. This morning (2 days later) I noticed that there was fish pool all over the bottom of the tank but it was all encased in sort of like cobwebby whitish stuff and upon further examination the sand and the decor was all covered in a light coating of the same stuff.

Does anyone know what i am dealing with and how do i get rid of it. Another concern of mine is. I used my net to take the dead fish out of the tank to flush it and while i was at work my roomate used the same net in my saltwater tank to remove a snail that had died. Will this harm my saltwater tank? The net was rinsed thoroughly but without any other precautions. I freaked out when he told me he used the same net!!!

 

have you checked your ammonia levels? the mater you see could be from the dkin of this fish itself.