One wall in our basement has a 'crust' (for lack of a better word) in spots. Kind of looks like wet flour dried on it. It comes off fairly easily, and the concrete below doesn't seem to be effected. The 90 year old walls are currently painted. The only water trouble we've experienced (been there 2.5 yrs - through one of the wettest winters/springs the area has seen) was when the gutter extension fell off, causing a lot of water to fall in one spot (10 inches of snow melted because we had a 60 degree day with RAIN, RAIN, RAIN, followed by another 12 inches of snow a few weeks later - not a fun year).

Any thoughts on what it is and if we need to get it fixed before finishing the basment walls?