Cucumbers are perfectly okay for turtles to eat, along with other things like flies, worms, snails, and even small frogs. Although you will have to feed them guppies, it is also acceptable to add in the occasional cucumbers.
As long as you limit the amount of cucumber you give to your turtles, they shouldn’t have any problems at all in terms of their overall health.
Keep in mind that turtles also need lots of leafy greens which contain a significant amount of vitamin A up to four times a week. While some turtles will not like the taste of cucumbers, others will gladly eat them.