IME, cucumbers don't seem to set off the anemone's stinging mechanism for some reason. I have cucumbers that will lay under my haddoni all the time. This haddoni eats everything else in the tank; hermits, snails, fish, shrimp, whatever it touches, except the cucumbers.
If the purple cuke was a sea apple, I would leave it at the LFS. They are difficult to feed and can poison your tank.