They will start eating from the middle/inside of the cucumber which is softer, and work their way out to the harder part of the cucumber.
Hmm...not sure what's the nutritional fact like, but I guess they can't thrive solely on cucumber, so this is just a treats for my suckers.
On the other hand, I do feed algae to my suckers....maybe make a video how I feed algae to my suckers in the future, in the meantime, enjoy this video :)