Goldfish guru,
If the pleco was sucking on your goldfish and now it is clamping its fins then clearly it is in considerable distress and you need to remove the pleco immediately. Surely you don't need us to tell you that?
If you have a look in the Compatible Fish forum you'll see a plain warning against putting common plecos in with goldfish for exactly this reason (and also because plecos grow huge). In future, if you don't know
exactly what species of fish you are buying, then don't buy it - you don't know how much damage it might do or even what its particular care requirements are.
I would suggest taking the pleco back to the store and putting a little Melafix in the goldfish's tank to aid healing of its coat.