I made white cakes last weekend and saved the 6 egg yolks that I didn't use. I put them in a plastic container with a lid in the refrigerator.

Now I need to make custard. Do you think I can use those yolks that were covered in the fridge for a week?

I don't want to waste them but don't want to get anyone sick. What do you think?

Probably not. Yolks get dried out very easily, I wouldn't chance it. Next time cover with a little water and they'll last longer or you can even freeze them.

Here you go:

Eggs

All eggs must be refrigerated.
Eggs with cracked shells should be discarded.
Because raw eggs are sensitive to temperature variation and movement, the egg holder on the door of your refrigerator will encourage spoilage.
They will keep up to 5 weeks refrigerated in their original carton. Eggs that are hard boiled and left in the shell can be stored for 8 to 10 days.
Egg whites should be stored in a covered container, egg yolks must be submerged in water in a covered container. Both will keep for 2 to 4 days.
Do not freeze eggs in the shell. To freeze, they must be broken open and placed in a freezer container to which 1/8 teaspoon salt or 1 1/2 teaspoon sugar have been added for each 2 whole eggs or 4 egg yolks. Be sure to note both the number of eggs and what was added.
Thaw frozen eggs overnight in the refrigerator and use immediately.

HTH