If the metal was rusty when it was painted it will continue to rust under the paint; that could be part of the build-up under the paint. The dripping water can carry rust with it and make little clusters or it may be carrying some of the mortar with it and making little deposits.
Check for roof leaks, condensation on the underside of the roof that may drip down and check the roof overhang at the top of the window; the water may be coming through the brick wall to get inside. If the water is coming through the wall you may need to plaster the outside to waterproof it.
Please post back and let us know how it comes out.