Size will determine species. Most of mine have lived for four years plus, the big ones I believe are still going (rehoused during a move)
It is highly unlikely the filter is whipping away the doo doo before you see it. They do a good job but gravel cleaning and plecs go together like day follows night. When you vac you should see evidence of faecal matter from this fish.
It is possible that enough algae is being sourced from the tank glass etc. but in my experience plecs normally come out for extra dinner.
What is the rough size of the fish, length to tail, not including.
It looks rather like hypostomus, they get BIG! There are a few species it could be which will determine adult size. What size is the tank?
Most likely to be a parasitic or bacterial infection. Plecs poo and often.
Have you read the links I provided?