What species of pleco?
If it's one of the common types then yes, cucumber is fine to feed. Mine like zuchini, mushroom and green beans as well. Pretty much any vege is safe, just experiment to find what ones they like the best.
I feed mine plecos tablets, assorted veges and an occasional meaty treat (shrimp pellets or a whole shrimp)
They are omnivores and will eat most anything thats 'edible', but give them a mostly vege based diet for their long term health.
Cheers
Ian