Runny stool is often a sign of parasites. They can get parasites from crickets. Not changing the water frequently can cause it too. Stress can also cause runny stool, but having runny bowel movements several times in a row is more likely parasites. You should have a fecal sample tested to be sure. Some vets will let you drop off a sample to be tested without bringing the frog in (less stress for the frog). Hope you your little guy feels better soon!