I am not a sea horse expert, so honestly take my opinion with a grain of salt lol. they look healthy to me but i have never owned sea horses so maybe i do not know what to look for. i have heard that you look for concaves in the stomach area. maybe another angle for other more experianced people might help.
The darker male looks thinner but tbh they both do.
Are the captive bred or wild caught? What's your feeding regimen like? Other tankmates?
If you look at them straight on they should be slightly convex in the middle. Each segment in their body when you're looking broadside should also be slightly convex on healthy seahorses.
They are wild caught. I feed them gut loaded adult brine and ghost shrimp. i also offer frozen brine which the female and one of the males will eat. I feed them 3-4 times a day. The other tank mates are a scooter dragonette and clown goby.
I often find it hard to tell by the pictures but in this case they do look a bit thin. If they are thinner than the females then it would be a definite but it could be that the females are thin also.
As many of you know, I'm a BIG proponent of feeding more times a day so as to closer replicate what they might do in the wild.
It may be that you haven't had them long enough yet to fill out under your feeding regime, or, that whatever you are gut loading them with is insufficient.
It would be much better to get them on frozen mysis, not brine shrimp. Brine shrimp have plenty of protein but are lacking in DHA, and, closer replicate what they might eat in the wild.
The other consideration is that the amount you are feeding is insufficient as the tankmates will likely consume more than their share, being faster feeders.
For feeding four times a day, I feed about 3.5g of Hikari mysis to 7 H. barbouri as that is what they consume in about 10-15 minutes, 20 at the outside. However, I don't have tankmates, especially faster eaters.
I feed about 4g four times a day to my 5 H. abdominalis but they are a larger seahorse.
All my seahorses also get live adult brine once or twice a week that I grown on a mix of spirulina and Algamac 3050, and enriched before feeding on just Algamac 3050.
None of them will touch frozen mysis yet but i am still offering it. the other tank mates (scooter blend and clown goby) will eat them anyways, the frozen brine i am using to try to transition the to frozen mysis eventually.