Skin Shedding
African Clawed Frogs shed their skin as they grow and every few weeks as it becomes worn. You may see thins strip floating in the water or catch them with a mouthful - they naturally eat their skin once shed, it contains nutrients and antibacterial agents which are good for their health.
Healthy skin shedding should only take a few minutes and looks as though the frog is convulsing - don't panic. If your frog is shedding it's skin consistently or in small pieces it may be sick or the water conditions might not be good - investigate if this is the case.