It's just dirt on the edge of the lace. It looks seriously grubby - probably a mixture of old glue, sweat and environmental dirt. You could try just soaking it in alcohol for 20 minutes and then very gently rubbing it to see if that cleans it. If not, you will need something stronger. Some guys have recommended Autoglym as a miracle cleaner for old glue - it is formulated for getting engine oil off your hands and engine parts. I haven't used it myself, but it has been well-tried and tested for hairpiece lace.
In my view, as I have said before, the piece is also too dense. As I recall, you were OK with a somewhat artificial-looking level of density, but it's quite likely contributing to the glances you are getting. Your sidehair has retreated quite a lot, so a huge wad of hair like that, and a dense hairline, look unusual. There are people whose natural hair looks like that, but it does draw attention. If you are fed up with the glances I would suggest you lower the density and get a graduated hairline on your next piece.
Noah