wax worms or earthwoms are better than mealworms when trying to entise an amphib to eat. if you removed everything from the tank and cleaned the tank and the items in it , then it is safe to say that something in the tank is causing the issue. a piece of driftwood may have a fungus issue or something like that. check you water quality and try buying some bottled water and not using tap water to see if that makes a difference. what is filter media are you using and did you rinse it prior to using it? post some pics if you can of the tank with the slime to see if it can be identifed.