I had some black foam that turned white after just a few days. I never could figure out a cause from anything variables in the tank, so I decided it was cheap foam. This had been the first, and last, time I had used that type of foam and my best guess is that had been dyed to look better. It sure didn't seem to bother the shrimp at the time, but I didn't like the idea. Not saying this is what happened to you, but this is the only time I've heard of such a thing to such an extent.