To me it looks like a zucchini or squash. If it lengthens it could be a zucchini “Magda,” the color seems right. But I really think it’s a particular variety of squash (or a mutant) that is globular, called “cue ball.” I don’t know how to share a picture, so this link will have to do.
https://www.johnnyseeds.com/vegetables/squash/summer-squash/cue-ball-f1-squash-seed-3359.html
The seed catalog I am looking at shows a light green, yellow, and dark green version. The dark green one is sometimes called “Eight Ball zucchini.” Light green is called “Cue Ball” and yellow is called “One Ball.” Curcubits cross pollinate routinely, so it could also be some sort of mutant - congrats on your cross breeding program! :)