Tiny white dots about the size of a pinhead can be found in reef tanks, overflow boxes, and sumps as well as on return pumps. They are Spirorbid Worms, which are filter feeders. It won't hurt to scrape them off if they obstruct your view, but they are harmless. This image is of their selfmade shell, and a tiny featherduster head pops out to capture nearby particulates. Reef-safe.
The above image is a super macro shots to show you what they really look like, but to the naked eye they are a tiny white speck dotting your tank here and there.