Hi Emily If they are real RR ties, they are treated with creosote...an excellent long term preservative...but it too has a limited life. If you are ok with the way they look, replace them with other RR ties. Just make sure they are well anchored to the hill they are retaining. Plus I like to bore 1/2" holes in the first course and drive 1/2" rebar down at least 2 feet into the ground to hold them in place....two rods per tie should be ok.
The mildew is collecting where it is becuase it is warm, dark and damp. You'll have to scrub the area with a scouring pad, particularly the grout lines, which is where the molds/mildew/fungii usually hide out. Then once well scrubbed, you will have to apply a chlorinated bathroom cleaner to the area at least once per week with a sponge