I have no personal experience with Hippocampus kelloggi but over the last couple of decades I have seen my share of seahorses with skin erosions or open lesions such as you describe. Most often these are due to bacterial infections such as marine ulcer disease (Vibriosis) or white patch disease (Myxobacteria) or mixed infections, and require treatment with broad-spectrum antibiotics in isolation.
A picture would be most helpful in diagnosing the likely cause of your problem. If you would like to send me a photograph of the infected area below your seahorse’s dorsal fin, I would be happy to share my thoughts on this condition with you and to suggest a possible course of treatment. Send the photo(s) to me at the following address:
Best of luck resolving this problem, Ricky! I would be very interested in hearing how your H. kelloggi fare over the long-term. Do you know if yours are captive bred or wild-caught, Rick?