Looks like there is a "halo" around the plug which suggests excessive oil in the coverstock and as far as the white specs I would guess the plug set up too fast and all the air bubbles didn't get a chance to escape.

That looks like not enough hardener, or the room too cool and the plug not cured all the way.  My first thoughts...

When you finish sand the plug, do these marks come out?   I had a similar situation with Tech-Line plug and I was told that you have to leave a tad bit above the surface and work it slightly with a bevel knife and file.   Then use paper or your resurfacing machine.

I is probably mostly thumbs that do this and larger thumb hole sizes at that.

I generally saw this when the plug gets too hot and cures so fast the air bubbles don't have a chance to escape.

We did a couple of things to help this.

One was to take a bit slightly larger than the hole and drill down about 1/4" to give a clean edge with no bevel.

The second was on large holes we would only put enough plug to fill the hole about 3/4 of the way, leaving it down an inch or so. Then filling it to the top once that initial pour cured. That keeps so much heat from building up from a complete hole of curing plug.

Pinbuster gives good tips on plugging the holes.  Try those tips and you might find you get better plug work.

I never try to plug balls in 1 stage, even if the ball plug says you can do so.

Interesting.  It was a finger hole (had grips).  The thumb hole was OK, with no pits.  The small shallow "engraved name" that was drilled out overtop of the pin (shown in picture too), turned out clear, no issues. 

I tried using a "cube" of plug, but they were a little too big to fit into the finger holes.  I assumed the volume of the finger hole was small enough that it didn't heat up as much... 

Thinking back, I did plug the thumb first, then the fingers... day after the fingers, I cut both down.. maybe I needed an extra day to make sure everything cured properly...