As you sand through various layers of fill coats that have cured for different times, it is not uncommon to see some difference in the way that one layer sands vs another. As a result you will see something like what you see there.

It is possible that this is associated with some failure to bond between the various layers, but not necessarily. And even if the interlayer bond is not as strong as the surrounding material, it doesn't mean your boat is going to fall apart.

I'm suspect that if you wiped the area down with denatured alcohol the difference would disappear, and that if you over-coated with epoxy or varnish that you would not see anything in the finished boat.