'Fraid I can't help with the id on your critters, but I have some white spots on my tank walls also-I thought they were baby snails cause you can see a visible spiral to their construction. They are about the size of the head of a pin (the largest that is). I mentioned it to my LFS lady & she said they were corals!!?? They had some like them growing in their live rock/coral tank.
She said the biggest difference is they don't move, whereas snails would (they circled the ones on the front glass, then waited to see if they moved).
I can't hardly believe that, cause my water is nowhere near high enough quality to have corals growing in it, but since half my live rock is from her, who knows? There are some of them growing on the rock I got from her, also.
Any guesses?