Brake fluid is not corrosive as far as harming anything in there, all you have inside that dust cover is the chromed steel piston, the alloy body of the calliper the "rubber" of the piston seal and the "rubber" of the dust seal. Years ago (too long ago) brake cylinders used to have a smear of rubber brake grease inside the dust covers maybe to keep some moisture out, these dust seals on callipers must always end up with letting some moisture in there.
I think, but have never done this, that once these pistons start to get stiff, it is time to clean them up if possible and fit new seals as the seals are now hard and dragging the pistons back through them will damage the seals slightly.