Yes, sir — once you begin antibiotic therapy, it’s very important to use the correct dosage and to maintain the treatment regimen to the end in order to minimize the risk that strains of bacteria that are resistant to the antibiotics could develop.
For best results, most antibiotics are administered for 10-14 days, so I would maintain the antibiotic therapy for the full 10 days. That’s a good combination of antibiotics for such problems, and if the growth on her face doesn’t respond to the combination of triple sulfa and neomycin sulfate, there are too many other alternatives.
You certainly could add a formalin bath every other day to your treatment regimen to see if that might make a difference, but I would definitely complete the regimen of antibiotics as well. Myxobacteria can kill a seahorse quickly when the bacterial plaques involve the head of the fish, so I think the antibiotics are helping; at least they seem to be preventing the growth from spreading further…
Be sure to gradually reduce the water temperature in your hospital tank/bucket to as low as 66°F-68°F if possible, and use the increased dosage of the antibiotics that is appropriate for marine aquaria.
When administering antibiotics, the proper dosage for a marine aquarium is usually at least twice the recommended dosage for freshwater. In the case of neomycin, some seahorse keepers increase the dosage of neomycin sulfate up to four times the recommended dosage for saltwater tanks, or eight times the suggested dosage for freshwater (Keith Gentry et al.).
Best of luck clearing up the growth on the snout and face of your female, Seagazer.