crayons gras -- wax pencil, grease pencil
crayons de cire -- crayons

There's a difference between crayons gras and crayons de cire. I haven't really heard crayons à la cire used that much but presume that it's synonymous to crayons de cire.

But are they synonymous?

à la cire  --  made of wax.

crayon gras = wax pencils, perhaps the sort of thing that a stage actor would use ie. as makeup ?