Interesting info, I did not know this.You're probably aware and discounted this already but just in case, ADFs do shed their skin, they usually eat the sheds but is it possible that's what you found?
Interesting, I had heard of oto doing this but not pleco. Thanks for sharing.You can have a (long!) look through the pictures at Planet Catfish.
Does not look like a common- the nose on your fish looks wider, more rounded.
Could the frog have died first, then been eaten?
Some Plecos (and other sucking fish like the CAE) will suck on the sides of large fish like Angels and Discus to eat the slime coat. I am not sure which will or won't do this, but I have heard of Commons (lots of posts), Otos (1 post) and CAE (lots of posts).
What size is your fish?
Does it eat wood? Vegetables? Carnivore wafers or pellets (such as shrimp pellets)?