Did you use fresh charcoal, I had a marbeled cat do that once when I put in unrinsed charcoal, I took out the charcoal and did a 30% water change and it went away.
I have a lima shovelnose and they will shed once in a while, I think I have read that it is normal, or stress induced. I can't remember for sure I will do some more looking and see what I come up with. Hope all is well.
