The pouch of the seahorse consists of four thin layers of epithelial and connective tissue, and the pouch or marsupium will often change color as a male’s pregnancy progresses. Initially, the gravid male’s pouch will change from a light opaque color to a dark brown due to the elaboration of the internal structures and thickening of the walls of the pouch that occur as the lining of the pouch undergoes placenta-like changes. The pouch will enlarge steadily over the next few weeks as the young grow and develop, and the aperture will change from fully dilated to a tightly closed vertical slit. As the delivery date approaches and the pouch expands to its fullest, stretching the thin membrane of the marsupium accordingly, other color changes may occur to the brood chamber.
Zulu-lulus (Hippocampus capensis) are real bottom huggers by nature, and normally do not spend much time in the water column, so if your mail Zulu is pregnant, it’s not surprising that he’s orienting to the substrate. As long as he is eating well and able to swim normally when he wants to move from one hitching post to the next, you needn’t be concerned.
As for your male reidi, it is normal for a seahorse’s pouch to change color when the males are courting, brightening or lightening in coloration during their courtship displays, and the marsupium may also change color during pregnancy as discussed above. Ordinarily, I don’t worry as long as the whole pouch exhibits uniform color changes. It is small, localized areas of depigmentation or white patches that are troublesome. So my best guess is that your reidi stallion is fine. Keep a close eye on him for the next few days and see if he is actively courting one of the females at this time. That could explain the change in the color of his pouch.
Best of luck with your seahorses, Nigel!