This is a small, round, slightly flattened, light green squash. The average size is a bit bigger than a round plum (cirruela in Spanish).
It looks quite distinct from Round zucchini but it might be just a variety of the same group (Cucurbita pepo summer pumpkin group)
The only similar variety that I could find is this Ronde de Nice (AKA Gourmet Globe) but when googling it, I can only find images with Round zucchini.