Cucumber leaves are a good leafy green for a rabbit. The fruit itself (technically, cucumber is a fruit) is ok to feed in very small amounts once in a while -- as a treat.
Introduce only a small amount to start. Check that day and the next to see if poos change. If all's good then cucumber is something you can offer now and then but still only in small amounts (maybe about a 1/2 inch slice max on any given day).