Hi Lucien,
I have looked at the pictures several times and cannot see clearly what you mean by the white stain. Perhaps you can post a better picture here.

In the meantime, if the white stain can be easily washed off with soap and water then I would guess it is mold, mildew, or perhaps residue from the old cardboard box that the beads were stored in. If the stain does not wash off, then we need to see a good close-up picture to go further.

Welcome to the forum!