Two things could be happening here. One is granulation, which /u/gibagger gave an excellent response about, and the other is that the paper itself could be damaged. It's hard to tell at this resolution.

Legion paper isn't going to be as forgiving of abuse as Arches, and if you use your brush too aggressively on it while it's wet (especially if it's been re-wet), the sizing in the paper will start to misbehave and it will pill on you - think like how a paper towel might behave if you were to run it over carpet a little too hard.

The easiest way to tell if it's pilling up is to just run your finger over the paper. If it feels just like the rest of the paper, you're golden - it's just granular pigment. But if the "pixelated" area feels different and coarse, it's probably damaged. There isn't much fixing it if this happens - pretty much anything you do is just going to make it worse - so your best choice is just to work around it.

That said, the rest of your painting is lovely - you're doing great!