Many species would be happy at that temperature but they should be in the ocean for it to be best for them.
Seahorse keepers usually keep the temperature in the range of 68° to 74°F because the seahorses are extremely susceptible to bacterial influences.
These nasty bacteria multiply
exponentially with each rising degree above that 74°.
The lower temperature coupled with more extreme tank husbandry than a reef tan gets, help to lessen the chances of damage and death due to bacteria.
While there are tanks out there successful at higher temperatures, many many more have failed in the attempt to do so.
My Thoughts on Seahorse Keeping
ps While I don't have first hand experience, over the years I've seen that some sps corals don't fare well with the "dirty" water created by seahorses like the particulate matter expelled from the gills upon snicking up their food.
I don't remember off hand which ones seem to suffer from this.