I can't tell absolutely from the picture.
Do you have the seahorse in a tank with stinging corals? Unprotected heater?
I've not heard of any problem with snails and no experience with crabs.
My first thought was flesh erosion often caused by a vibrio species of bacteria.
This is a water quality issue usually with trapped unseen uneaten food and detritus proving bedding for nasty bacteria like the vibrios and often exacerbated by temperatures above 74°F. Also can happen with mechanical filters of various kinds that haven't been cleaned often enough to remove trapped food/detritus.
In any case, I'd place the seahorse in a 10g hospital tank with aeration provided by open ended air lines, dropping the temperature slowly to 68°F.
If it is a flesh erosion disease then treatment with antibiotics will be required.
I've used Furan II for this problem, often successful but not always.
You can also use tri-sulpha at the same time.
Hopefully others will chime in and have experience with the same wound appearance.