I have never had this happen but I suspect it is from the dead material (old roots ect) Being the medium is inorganic, then it must be from dead living material. What I would try is doing a Physan soak followed by a thorough rinse. Cover the drainage holes...soak 15 or 20 mins...drain and rinse thoroughly a few times.
Being I am not an expert on this, get other opinions prior to trying my suggestion. I believe it is what is called "snow mold"