?
OMG, that's the biggest hole I have ever seen on a shrimp.
What shrimp was it? Cherry red?
Did you other shrimps also have brown spots?
And did it have other brown spots in other locations on it's carapace (shell)?
On first diagnosis based on the one photo above, it is most likely Chitinolytic bacterial disease.
And if you have Cherry shrimp in a tank with low 5.5pH, the shrimp might have weak immunity due to keeping it in a pH range outside it's comfort zone. Cherry shrimps do better in 6.8 - 7.0 pH.
What type of water do you use? RO? Tap?
If you use RO, do you remineralise it with Calcium/Magnesium?
I see you have a bare bottom tank, so it's not planted substrate that is pulling the pH down.
Are there any more shrimps in the tank?
Sorry for the questions, but we need to get an idea of your situation in order to give you any advice.
LOL, we post very similar questions within seconds of each other.