Very sorry Cait for not coming back to this thread sooner. I had forgotten to check back. From the damage and the behavior that was already present, I doubt the shrimp is still alive now, but hope this info may still help.
I looked up this shrimps water preferences, and the GH, although on the low side, it shouldn't be a big problem. So now I don't think it's damage from too acidic water.
Take a look here
Shrimp Disease? Shell rot? - Care, feeding and breeding - The Shrimp Spot
It does look like it is the disease mentioned.
Seeing as how the disease is a anaerobic gram-negative bacteria and is similar to Vibrio spp., Pseudomonas spp., Aeromonas spp., and Flavobacterium spp., a antibiotic that treats those same bacteria in fish, might work (should, but there's no real mention of a cure for inverts) for the case of your shrimp's bacterial infection.
Antibiotics that typically effective treatments to kill anaerobic gram-negative bacteria are Kanamycin (found in Seachem KanaPlex), Nitrofurazone (API Furan-2), Minocycline (Mardel Maracyn 2 -NOT 1-), Oxytetracycline is not as effective as Minocycline, but is better than Tetracycline, but either could be used, just varying success chances in curing), and Triple Sulfa.
On this study of the disease they mention treatments/cures to stop progression of the disease and I assume upon the next molt the shrimp's exoskeleton should be in good condition.
On page 4 (page 10 of 51 for the pdf) is where it talks about possible treatments.
For this type of disease I think Kanamycin (Seachem KanaPlex) usually gives the most successful results. I am not 100% sure, but I do believe Kanamycin is safe for inverts. It's supposed to be safe for use on aquatic plants.
Minocycline (Mardel Maracyn 2, not Maracyn 1 which is Erythromycin and would not be effective) would be my next choice. Pretty sure it is safe for inverts and plants. A lot of meds don't list shrimp/plant safe because they have not tested it on all shrimp/plant species, but from reports by users, many do report it not harming their inverts and plants. I haven't used it myself so I can't comment from first hand experience.
Nitrofurazone is good, but not safe for inverts/shrimp/snails or plants, so that isn't a good to save the shrimp.
So get KanaPlex or Maracyn 2 and dose the tank. Increase water surface agitation and keep the tank clean.