Nothing is wrong it is a normal pattern on the leaves, very common with zucchini. Just ignore it. It will not harm the plant and you can't fix it. All the leaves won't have this pattern, it typically occurs when the plant matures and then all subsequent leaves will have this pattern.

Agree with Trev....this is normal. My cushaws and heirloom squash all have this pattern on their leaves.

I too thought it was mold developing when I first started pumpkin and squash gardening years ago.I took one of my leaves to local garden center to show them the weird pattern/mold?....was told coloring was due to leaf maturity.