Looks like a mosaic virus to me. Maybe cucumber mosaic virus (CMV) or squash mosaic virus (SqMV). Both are transmitted by insects. It’s difficult to tell from the photo but are the leaves wrinkled as well as mottled? CMV will cause this. It’s transmitted by aphids. You should have a look for aphids on your plants and try and wash them off, some warm soapy water will work or you can try the hose. If its SqMV you may see some yellow spots on younger leaves. This virus is transmitted by beetles rather than aphids. Either way, the bad news is that you cannot cure your plant of these viruses and the squash produced by the plants will likely also show symptoms. You can pull up the plants to try and limit the spread or you can try chemical control to kill off any insects. Good luck.