Not toxic, but not suitable to eat together.

Because the enzyme in cucumber destroys vitamin C in tomatoes. cucumber contains a vitamin C enzyme that destroys vitamin C in other vegetables. the more vitamin C there is in the food, the worse the level of vitamin C that is destroyed by the enzyme in cucumber.

Expanded information:

Vegetables such as chili peppers, cauliflower, celery, and bitter gourd also contain high levels of Vitamin C, it is best not to eat with cucumber. Vitamin C decomposition enzyme will then destroy the vitamin C of other foods, although not harmful to the human body, but will reduce the body's absorption of vitamin C. If you wish to supplement vitamin C through chili peppers, celery, kale, bitter melon and other foods, then it is best not to eat with cucumbers.

Reference:

People's Food Network food channel to eat fruits are also taboo, what fruits should not be eat together