Biopsy shows spitz nevus cells under another type of benign mole. melenoma can't be ruled out yet. can melanoma grow under a benign mole?

Yes: In fact, this is fairly common. No one knows where spitz nevus leaves off and a melanoma capable of metastasizing begins. This is one of the hardest calls in pathology, and some say it's impossible. The burning question will be whether to do additional surgery.

Yes-depending on ?: Melanomas can be found growing adjacent or next to benign moles. Not really "under". Not sure if this is what you are asking.

Yes: Things can grow on top of each other in the skin. Happens all the time. You can get malignant things on top of benign things, benign things growing on top of other benign things, etc. Melanomas can also be seen right next to or even in other benign moles.