I'd guess there is a tiny bit of movement that is wearing the color off the edges. It doesn't look structural to me, so I'd just hit it with the edge dressing and go from there. Just my (non-expert) opinion.
Well it's not just cosmetic. There is a small "crack" per se in the levels or stacks in the heel. Overall the heel still feels secure. I'm just wondering will I need a heel re-craft soon and why this is happening?