Welcome to the forum! Can you provide more details about your coral and system, is the brown stuff a gelatinous substance? Coral will expel waste that looks similar to the picture but it could also be bacterial issues.
It’s one of two things. If you spot feed your euphyllia usually a day or two after they will “poop” or this could be brown jelly. I do not believe this is brown jelly though as usually the entire polyp will turn to brown goop where as in the picture the polyp is this in tact