There are some good ID sites out there with info on these kinds of things.
I use Melev's Reef and Xtalworld.
This is the blurb on Melev's Reef about them...
Tiny white dots about the size of a pinhead can be found in reef tanks, overflow boxes , and sumps. They are Spirorbid Worms, which are filter feeders. It won't hurt to scrape them off if they obstuct your view, but they are harmless. Reef-safe.