Leather Collar Mold Issue

I’ve got a good quality leather pronged dog collar that I bought some years back. Recently, I spotted some faint powdery mold growing on it. I treated it with a weak bleach mix, let it sit for a while, rinsed it off, and left it to dry out. However, the mold seems to be returning. Does anyone have suggestions for getting rid of it once and for all? I’d happily pass it along to one of my dogs if they would actually wear collars. Maid Lauren