It looks like the condensate drain line has splashed up sediment rust onto the window frame. Can the rust be removed and with what?
Some CLR and call it good. I think you should note that the problem will continue unless corrected somehow.
Use sno bol toilet cleaner .
mild soap & water 1st
eifs finish may bleach/discolor
may have an undesirable affect/repercussions on thermal window swiggle/desiccant adhesives
Rubbing compound (for automotive paint)