A few weeks ago I posted a similar question to yours - because I have the same problem. The person who previously responded to your post is correct, it's just mineral deposits. Really the only way to get rid of it is to scrub it off in water, and then keep the area dry. You can't use any cleaning products or soap for fear it might get in the tank (if you have fish it'll kill them). It's really nothing to worry about, just unsightly, so whatever you decide to do is up to you.
I dont know if this helps, but i use water and my fingernail if it is bad. I usually try to keep up with it, but in the beginning of fish keeping, i had no clue what to do, so i ended up with alot of crusties on my tank top. I took it off of the tank, and scraped it with my fingernail and most of it came off. Sorry i could not be of more help. Sandy
I had the exact same problem. I used lime away and everything. Best bet is a razor blade and bleach. Just be sure to clean the tank out well before you use it.