Hi Edwin,
I have never seen that in my babies. I keep them on cypress mulch exclusively. I soak it a couple times a week when it dries out. Really soak it, just pour the water in like a flood in nature. But I only leave them in the drinking water/bath long enough to drink each day. Those spots are about at the height of the water you soak them in, I assume, and it may have something to do with it. I throw whole greens(romaine, kale, escarole, etc.) in for them to eat and hide under to stay humid.
Wouldn't hurt to treat those spots with providine.
Hope you are still happy with the babies you got from me.
Russ P.