When I got my new tank, I left the old tank with about 2inches of water and about half of the old gravel in it for about a week. I noticed those little worms as well, and found out they were just nematodes. They're caused by too much feeding in the tank (overabundance of nutrients). I'm guessing that's why mine didn't show up untill the tank sat (no fish left to clean up the little bits of food still in there probably). From everything I've read/heard they're cocmpletely harmless, free living worms (non-parasitic) at least the ones that I found in my tank. They were small and white, exactly as you described.