Malachite green is an effective treatment for Cryptocaryon irritans and many other ectoparasites. Seahorses typically tolerate all the usual chemotherapeutics when administered at the proper dosages, including malachite green, but that is one medication I would not recommend using as a concentrated dip for seahorses. (Dr. Amanda Vincent has reported that Hippocampus can be sensitive to concentrated doses of malachite green.)
I would say that a freshwater dip is a safer alternative for you, Dave, particularly if only one seahorse seems to be affected. If you suspect the entire tank may be infested with the wormlike parasites, then treatment with a dewormer such as fenbendazole (Panacur) would be appropriate, providing the tank houses no sensitive invertebrates.
Best of luck treating your seahorse’s worm problem, sir!