Sorry about your loss. Frogfish should not shed their skins like Hawaiian leaf fish do. Possibly bacterial infection with skin sloughing? Or algae on the skin coming off?
I've had my painted frogfish several years, I think it's been three. I raised if from a 1.5 inch baby. I had his mate (they spawned) about three years as well, but lost her last year. I have had several frogs die suddenly for no apparent reason.
IME, the following help with frogfish longevity:
very good water quality
moderate to low flow
appropriate or no tankmates
a high quality, varied diet (getting them to eat frozen is important)
temperatures not above 80 degrees F
avoid changes in the tank - keep things stable