I can't really make heads or tails of the photo, but if the piston isn't retracting, I'd pull the wheel out, remove and inspect the pads, and gently lever the piston back in to the caliper (using a bicycle tire lever or anything wide, flat and rigid enough).
If the pads are thin - like almost as thin as the spring clip - I'd replace them and hope that it solves the problem (which may have been that the piston was just getting out towards the end of its range and getting ****-eyed).
If the pads are thin - like almost as thin as the spring clip - I'd replace them and hope that it solves the problem (which may have been that the piston was just getting out towards the end of its range and getting ****-eyed).