I could be wrong. But that looks to me to be damage from heat/burning. Is this plant in a VERY sunny window? Sometimes a plant will burn if the leaves get wet and then hit with very strong sunlight.

I rarely consider starting squash indoors. Usually, time gained by starting indoors, is then lost by transplant shock and the need to condition the plants to UV light.

You could try planting this squash outdoors. But work it up gradually, so it doesn't get sun burn. One hour for one day, 2 hours the next, 4 hours the following, then 6. After that you should be able to transplant it into the garden.

At the same time, for an experiment, you might plant a seed or two, directly in the garden, the very first day, and see which does better.

George
Tahlequah, OK