A yellow layer is an intersiting observation. I would usually expect white (from LiO2) or black (from Li3N). Depending on your application, a thin layer can be just scratched off (e.g. with a tweezer or scalpel). In any case, you should talk to your supplier about the quality of the product.

Steel wool is good for thoroughly removing a layer of oxidation.

Thanks for all of you

really? but Li3N is not black