Cucumber is a Green food. Green foods are safe to eat as a regular part of a diet.
Guideline related quotes related to Non-Starchy and Green throughout Geoff Bond's publications.
Eat at least 2 3/4 lbs of mixed salad and vegetables per day, consisting of "Green-Green” and “Green” foods. Of these, at least 3/4 lb should be mixed salad. Also include at least 5 cups of “Green-Green” leafy vegetables or 2 1/2 cups of other vegetables per week. ~Paleo in a Nutshell p.102
Specific references to Cucumber throughout Geoff Bond's publications.
It is quite easy really to achieve six servings by eating one big salad every day with all the usual ingredients we think of as salad vegetables: Tomatoes, lettuce, radish, cucumber, mushrooms, onions, and so forth. ~Paleo in a Nutshell p.48
Conforming non-starchy, colored plant foods are foods that are low-glycemic, rich in micronutrients and fiber, and harmless with regard to anti-nutrients and antigens. Broadly, they include most salad foods, such as lettuce, onions, cucumber, radish, and mushrooms, and they also include colored vegetables, such as broccoli, green beans, bell peppers (sweet peppers), and Brussels sprouts. These are considered "Green-Green," "Green," and "Green-Amber." Under "Green-Green," we have separated out the vegetables that have the high concentrations of background micronutrients that our ancient ancestors delighted in. You can have unlimited consumption of these foods, and the ideal is up to two pounds (900 g) per day. ~Paleo in a Nutshell p.62
The idea of eating salad for breakfast does indeed run counter to our Western cultural programming, but it is something that many societies do, notably in Africa. A copious mixed salad with some avocado, tuna flakes, or shrimp makes a great start to the day. Again, make it a large portion— at least one pound per person. It is not really so much: One large tomato, one cucumber, some onion, and some lettuce leaves make 9 oz of plant food. Round it off with 3 oz of canned tuna, and you have a hearty breakfast. ~Paleo in a Nutshell p.87
The vegetables from above ground cover a huge range of plant parts: stems, such as asparagus from the Mediterranean and kohlrabi from Europe; buds, such as Brussels sprouts from Belgium; leafstalks, such as celery from the Mediterranean and rhubarb from Asia; leaves, such as Europe's cabbage, lettuce, and spinach; immature flowers, such as cauliflower from Europe, broccoli from Turkey, and artichoke from the western Mediterranean; immature fruits, such as eggplant from southern Asia and cucumber from northern India; mature "vegetable-fruits," such as tomato from Peru, avocado from Central America, and bell pepper from the Andes; edible bean pods, such as runner beans from tropical America; and edible fungi (mushrooms) from just about everywhere. Of course, today, these plants are grown all over the world, wherever farmers can produce them economically. ~Deadly Harvest p.56