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 purple stuff is coralline algae branching out on the glass. And below are some super macro shots to show you what they really look like.