Have you tested the water for ammonia, nitrite or nitrate? If you have these in unsafe levels it would be a good start to treat the water for that.
You might need to do some vac'ing, scooping, reducing excess decaying leaves, muck if it is there. Some water change. Not sure how you do maintenance on your pond.
From the pic, I cannot tell what it is.
Getting the water quality in a safe zone is one of the first line measures to help sick fish.