Yes Cucumbers are fine for a tortoise to eat and is part of a recommended diet for Tortoises.
A Tortoises feeding Diet should consist largely of leafy greens and grasses, supplemented with a variety of other vegetables such as broccoli, cabbage, cauliflower, Cucumbers, carrots, etc.
And fruits that you can feed your Tortoise are apples, apricots, grapes, melons, peaches and strawberries.