Hold The Phone!!!
Well, after months of searching in vain for info on this scale-looking parasite, I just read a post from sarahbobarah in the shrimp forum section concerning olive nerite snails. It seems that what I thought was scale is actually olive nerite eggs! Now, these don't look like regular snail egg clutches. The eggs are sort of half round, white, and are deposited in a line, usually along a stem or crease in the rock or wood. When they die, and/or are eaten, there is left a little white circle, like a teeny, tiny atoll.
I understand that they will not hatch without a brackish environment. But at least I know now that this is not a parasite.