Assuming the card is completely dry now, your chances depend on what and where the "white stuff" is. If it is a conductor and crosses any electical paths, it could be very damaging. An if any seeped into places you cannot conveniently reach, it might be irreversible. On the other hand, if it is not "hidden" and you clean it, you might not have a problem.
I suggest first using canned air to blow off any loose particles and to try to reach in and around all components. The use a damp, lint free cloth to gently wipe it off. If any remains, even a slight film, I suggest trying to clean it with alchohol on a clean, lint free cloth and following that up with a damp cloth (yes - all cloth should be clean and lint free) to remove any lingering traces of the alcohol.
If you can't get it clean, just sell if on eBay (just kidding). If that does not work, you have to decide what risk, if any you want to take. You can try it as it is, with some risk to the card and perphaps (less likely) your system, or try to clean it using the same process but subsitituting vinegar for the alchohol. Vinegar can be used to clean hard water stains on some surfaces, but I don't know if it would work on the card nor what damage if any it would do to the card. Vinegar is slightly acidic, but I don't THINK it will do any damage.
After cleaning you should try it in a friend's computer first (again just kidding).
If it appears clean, I would try it - but you still don't know if there is any hidden damage so there is a small risk. If you try it, you might want to have the computer case open so you can observe - and perhaps smell - if any problems arise.
I think you should be OK if you can clean all apparent residue, but cannot offer any guarantees.