What is this white layer on brass?
Judging from the location and colour, it is most likely residue from liquid polish that was not properly rubbed/polished off. Quite a few types of liquid polish for brass turn white when they dry, and any folds and creases in the design is where they would be most likely to accumulate if the person doing the polishing is not very thorough.
Depending on how long its been there, you may be able to shift it with just warm water and a soft brush or cloth, leaving the patina mostly in place.  Otherwise you may need to use brass polish and polish the whole thing.
You could try asking advice from some of the better antique dealers or ideally someone who specialises in conserving antique brasses if the piece is a valuable antique.