If you use an English (aka "seedless") cucumber, you definitely do not need to peel it or remove the seeds before adding it to this salad. English cucumbers have thin skins and minimal seeds, so it's really the perfect cucumber for this type of salad in my opinion. And, I think the darker skin actually looks very attractive and adds contrast to the salad. You can check out this article for more info on English cucumbers! However, if you have another type of cucumber on hand, you can definitely still use it! You may just want to peel it if it has a thick skin (or not, this may be a matter of personal preference). Likewise, if the cucumber has a lot of large seeds, you may wish to scoop the seeds out since they can be bitter and add excess moisture to the salad. Once the cucumber has been cut in half length-wise, just scrape the seeds out gently with a small spoon.