Now that your tank has been up and running for a while, and you've added some fish and corals, you may have noticed some tiny white dots about the size of a pinhead growing on your glass, overflow boxes, and maybe even in your sump. They are Spirorbid Worms, which are filter feeders. It won't hurt to scrape them off if you don't like the appearance, but they are harmless and are considered normal in a healthy reef tank environment. This image is of their selfmade shell, and a tiny feather duster which pops out to capture nearby particulates. Totally reef-safe.