That filter is a little undersized for a 55-gal
- it will have trouble moving the water enough to trap that poop. Plecos are really prolific waste producers, so you might want to consider additional filtration, like a canister filter.
I don't know if your filter has this feature but on some you can adjust the flow of the filter by raising and lowering the intake tube, and maybe yours is raised up a bit, reducing the flow, so make sure it is pushed down all the way. That is a remote possibility, but worth a try.