if you are certain it's a cucumber, i'm fairly certain the seeds need to be taken out along with the "placenta".
unlike zucchini, the seeds of a cucumber are very tough and would most likely be difficult for the goldies to digest.
(we don't even digest them very well...)
but as previously mentioned...
heating in the microwave is quick and easy.
so if you wanna feed the cucumber,
peel the skin off (i'd leave a small part just so the veggie clip has something to "hold on to"),
scoop out the innards,
nuke it in the microwave,
let cool,
and you're good to go.
i've never fed cucumber myself, but i find that my goldies LOOOOVE zucchini.