It could be a parasite, or if the stool is expelled in moist conditions, it looks wetter than it actually is. My male ABF, Gherkin, had liquid stools when I first got him, and I realized that I was feeding him in his water dish and he gulped large amounts of water along with the worm. Everything came out a bit on the wet side! Once I moved him to a feeding bin, it was fine-though smelly! I frequently check the stool to ensure no parasites (especially in new addtions). Another factor could be a difference in diet. If a frog is fed the same meal day in and day out and then something new is eaten, it might set the digestive tract off a bit. Is your frog okay?