If you can scrape a bit off, and put it in a glass with vinegar overnight, that would tell us whether it's calcium carbonate based -- it will dissolve overnight if it is. I'm pretty sure I know what it is, but can you go ahead and send me some pictures? (Send to poolforum AT gmail DOT com)

If it's what I think, you can go ahead and refill, and start your pool. I can help you get rid of it over the next few weeks, without damaging your pool. You do NOT want to use a heavy brush, or acid, on a vinyl liner.

Ben