I'd be sending that photo to Metzeler pal
Lets face it, if it's nowt serious then carry on as normal
But if it's something they've not seen before, it could be important
I'm obviously no "tyre technician" but I've not seen that effect before
On ANY tyre over the last 50 odd years
It does look as if there may be more than one compound or maybe what we see is the "edge" of a layer
Either way I'm sure "peace of mind" is what you want
So send a few pics to the technical bods at Metzeler and see what they say
Keep us posted with your "investigations" bro