It’s very difficult to say what may be going on with your male Hippocampus erectus, sir. When you say you are concerned about a white pus-like substance coming from his belly, I think that what you may be seeing are white, stringy feces being eliminated from the seahorse’s vent. When a seahorse has been of its feed, rather than producing the normal fecal pellets you are accustomed to seeing, they will often produce white, stringy feces, which may be mucus coated.
The seahorse’s vent is the cleft formed by the combined openings of the anus and urogenital pore. It is the simple recessed passage located just above (cranial to) the anal fin in females; in males, the anal fin is located in the middle of the vent where it separates the anal opening from the urogenital pore. The male’s anus is therefore situated above the anal fin while the genital opening of the male is located below the anal fin at the mouth of the brood pouch. Three things are thus expelled through the vent: fecal pellets, urine, and gametes (ova or eggs, in the case of females, and spermatozoa, in the case of males).
So, considering where the vent is situated, white, stringy feces being eliminated from the body could easily be mistaken for white pus being expelled from the seahorses belly. If that’s the case, it doesn’t indicate an infection. Rather, the white stringy feces could simply indicate that the seahorse hasn’t been getting enough to eat lately. Or, they could indicate that your male is carrying a heavy load of intestinal parasites, which can sometimes cause a loss of appetite. In the case of the latter, treatment with metronidazole administered orally is often very effective in resolving the problem.
But if your male is indeed pregnant, Brad, he would be very near his delivery date. (Gestation in Hippocampus erectus is typically between 14-30 days, dependent largely upon water temperature and diet.) Gravid males often go off their feed as their pregnancy progresses and may miss a meal or two. They may also sit on the bottom and appear agitated and distressed when labor is imminent. So it’s possible that your male is just acting "broody," in which case he may be good as new again as soon as he delivers his young.
So whether or not your male is ailing and requires treatment depends on whether he is pregnant and producing white, stringy feces lately because he has been of his feed, or whether he is having a difficult pregnancy with complications that have delayed delivery (stillborn young perhaps?), or whether he merely looks pregnant because his pouch is distended with fluid or gas, indicating an illness or infection.
As a first-hand real-time observer, I’m going to need more detailed information from you regarding your male’s behavior and symptoms before I can get a clear picture of what is actually happening and how you should respond. So please keep me updated and let me know what you think. Any additional details you can provide will be helpful. Try to get a closer look at that pus-like discharge the next time it happens and let me know if you think it is actually a purulent release or may simply be white stringy feces.
Best of luck with your out-of-sorts male, Brad. Here’s hoping he is just experiencing some labor pains and a concomitant loss of appetite and will soon be good as new after delivering a brood of healthy fry.