About six months ago, I built a reclaimed, rustic-ish dining table from reclaimed yellow pine. It is finished in minwax spar urethane (3 coats). The other day, I noticed some black spots along with some grayish "fogging" in one particular spot on the table top. The affected area is underneath the finish - not on top of it. I don't recall these spots being there when building the table. It is a rustic table so it may have gone unnoticed for some time as it was partially under a place mat at a spot that never gets used - and it also blends in with the rustic nature of the table in general.

Here is the spot I'm referring to.

Screen Shot 2018-02-12 at 7.16.56 AM.jpg

At this point, I'm not even sure what it is. Is it mold? Could it be something else? What do I need to do to fix it?

Thinking it was mold, I researched online and found an article suggesting a solution of 20 parts warm water, 10 parts bleach, 1 part dishwashing detergent. I applied that to the area and let it air dry, as suggested, overnight. That did not seem to make a noticeable difference.

Can anyone help identify what I'm looking at and what I should do to fix it?