Sounds like a fungal infection. Very common in captive muddies.
Despite changing water every day, they may still get it.
Fungal treatments for tropical fish work well--especially tetracycline and methaline blue.
But the trick to keeping it away for good ( besides keeping the water very clean)is to LOWER THE PH OF THE WATER. Low PH is a natural retardant for fungus.
Also---and I know you know this already---feed them in a different tank or tub to help keep organic waste from their living quarters.

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