Hate to say it but after seeing the picture it does indeed look bacterial. I think what is happening, is that for whatever reason, they are not happy and dying and you're seeing the opportunistic bacteria on the spots of death. What kind of lighting do you have? I've burned GSP to death before not acclimating them to halide lighting. Try moving them down as far as possible with a medium amount of flow.
The dips we tried wouldn't work if the tissue is indeed dying as the bacteria have a seemingly infinite supply of fuel until the mat dissolves.
John