Mottled squash could be caused by a couple of things. 1. it could have been cross pollinated with a green squash. This can happen when the two are near each other.

2.mosaic virus -  The squash mosaic virus is transmitted to your plants by insects which have been feeding on other virus-infected squash plants or perhaps some wild plant. Once the plant gets this disease nothing can be done. Best preventive measures include insect control and planting varieties which will mature early in the year. This disease is more severe on late-planted squash. The squash is still good to eat if harvested at the proper stage of maturity and looks healthy. There will be little change in taste. This virus disease will eventually kill the plant.
Weed control and sanitation are very important since weeds and plant debris can serve as sources for viral infections. Clean up and throw away all infected plant material at the end of the season.