Dust particles in water

Why isn't my filter filtering out these dust-looking particles? This a day after a water change.

I think it could be dust falling in the tank or other materials.I wouldn't worry to much about it.But if it reallys bothers you filter floss should get it out.

I have a lid on the tank so I don't see how dust could be falling into the tank.

Happens to mine even though I got a lid.Like I said unless it bothers you,it should be fine.If you wan't to remove it filter floss.

These particles are on the surface, correct?

If so, they are sitting on top of the water, due to the surface tension of the water and never circulate through the tank. That's why they aren't being taken care of by the filter. Disturbing the surface a bit more with your filter or airstone would likely do the trick.