i think you might wan't to check on the stool.
Slimy stool is mostly a sign that the frog has a case of worms or other parasites.
Just try and collect a fresh bit of stool and put it in a small film canister with lightly moist cotton wool for example.
Check which vet is able to examine amphibian deseases.
Since all frogs posess worms in some way, it's best to let it check by an experienced vet.
They can diagnose the problem and subscribe a treathment to you.