Mustangs and Sunbursts (Hippocampus erectus) will often brighten in coloration, usually to a pearly white or cream color (with the exception of their heads and dorsal surface, which typically remain quite dark) during courtship and mating. So it could be that he is not yet gravid, but is performing pouch displays and has inflated his marsupium with water for that purpose, while he tries his hardest to get himself pregnant, and that may be what all the flirting and courting is about.
On the other hand, pair bonded seahorses also go through the same brightening in coloration and courtship displays during their daily greetings, so the change in coloration and flirtatious behavior you have noticed could also be an indication that the male Sunburst has mated and paired up with one of the females. The courtship ritual you observed today may simply have been the greeting ritual mated seahorses perform for one another each day in order to strengthen and maintain the pair bond, and in that case, the stallion may already be pregnant.
Either way, it sounds like he is a healthy seahorse with an active interest in courtship and breeding, so I would suspect that he’s just fine. My best bet is that your new Sunburst stallion is either already pregnant or very determined to get that way.
Best of luck with your new seahorses, Tammy. Here’s hoping that this time you have a healthy, young, an aggressive Sunburst stallion who is determined to lose his virginity and present you with a fine brood of young.