Is it coming from the shrooms mouth? These corals typically expell their "guts" then retract them. I don't know why they do this, buy mine do it probably once a month. It is very bad to add the salt directly to the tank. My guess is that some of the salt got on the shroom and began to burn it. They excrete a slime to try and shed off whatever is bothering it. Next time, mix the salt in a bucket, aerate it for a few hours, test it, then add to the tank. How can you be sure he put in enough or if he put in too much salt? You are lucky you didn't lose all your corals.