Is this flux residue or the PCB is burnt?
This board was probably manufactured in a part of the world where the labor rate is low.  As a result, manufacturers have more flexibility in trading off costs in getting everything right up front versus having to fix a few things afterwards.

Your pictures aren't very good, but it looks like flux residue.  This is common when hand rework is performed.  If this is rosin flux, which is sortof looks like, it's totally harmless.  It's basically like epoxy or low grade amber.

Does the product work correctly.  If so, I would leave it alone.  If it really bothers you, try swabbing the area with a cotton swab dampened with isopropyl alchohol.

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