How do you determine if a Pleco is male or female?
I'm afraid some plecs do develop a taste for the slime coat of other fish's. It's very hard to break them of the habit, once they've started, and it will cause illness or injury to the parrots.
All you can really do is move the plec to a tank with fast, slim bodied fish it can't attach itself to
There's not really enough nutrients in cucumber to make it worth while feeding. You could try peas (lightly cooked and shelled), carrot (also lightly cooked to soften it) slices of apple, mango or melon rind (with a little bit of the flesh left on) or blanched lettuce (not iceberg though; there's no goodness in it, same as cucumber).
Your parrot's slime coat will soon mend, once the plec is gone. It's not a very common problem, plecs usually are very placid.