Mucus in the urine can be bad. Next time it happens collect the mucus onto a paper towel that is damp and stick it in a ziploc bag. Stick the sample in the fridge until you can get it to a vet for testing. Before 24 hours passes is best.If its a very small amount and white its usually not a bad thing, but if its pinkish or red you'll need to get your frog some meds.