Get some leaf liter and pour it in on top of the substrate. Your frog will walk on the leaf liter instead of the substrate and not get so dirty which willl then allow him/her to stick to the glass. Leaf liter will also help with humidity and will naturally break down in the tank.

I get Sea Grape leaf liter from Josh's Frogs - Largest online herps feeders and reptile supplies store and he has a 5.00 shipping price which is very reasonable.