I can't tell myself if it is just normal with the colour change or not, but it DOES look to me to be quite thin and that IS a concern.
I know many hobbyists differ on this point, but I personally feel that seahorses need feeding more often than twice a day for best chances of success.
I used to feed 3 times a day, but now do it four times a day. My barbs aren't much bigger than yours, some same size, and I would say they are ALL eating at lest 5-6 mysis each of the four times fed per day.
Is this colour pattern on both sides of the seahorse or just the one? If just one, I might be tempted to place it in a hospital tank and treat with an antibiotic for the full duration recommended for whatever antibiotic you choose to use.
If it is a bacterial infection then it likely started before you got the seahorses, having only had them now a week.
While it is nice to have "parameters" in check, the problems that cause bacterial infections cannot be tested for with any test kits available to the hobbyist.
The problem usually stems from seahorse eating habits where they are selective feeders, leaving uneaten food particles that can get trapped unseen and not all taken care of by cleanup crews. This provides food and bedding for the nasty bacteria that can do so much damage to our seahorses.
Also, when the seahorses snick up their food, the masticate it and then pass it on to their digestive system, but on the way, particulate matter is passed out through the gills to further degrade water quality.
You shouldn't have this problem at this point so that is a greater indication that IF you DO have a problem, it was started before you received them.
Because the deteriorating water conditions can't be tested for, and, it is most times a fairly slow process over weeks to months before something is noticeable, I personally prefer to do larger and more frequent water changes, being extra diligent in ferreting out hidden detritus and uneaten food.