Yes, sir, treating your aquarium with fenbendazole (brand name Panacur) does limit the type of corals that you can subsequently keep in that tank, but fortunately the mushroom corals you are interested in don’t seem to be sensitive to the medication.
Fenbendazole does not have any adverse effects on biological filtration, but it is deadly to many Cnidarians besides hydroids. Mushrooms and related corals are generally not affected, but expect it to have dire effects on other corals (e.g., sinularias), polyps, gorgonians, and anemones. In general, any Cnidarians with polyps that resemble the stalked family of Hydrozoans are likely to be hit hard by fenbendazole, so that’s a good rule of thumb to go by.
If you want to be extra safe, or you are considering corals other than mushrooms, both activated carbon and polyfilter pads do a great job of pulling medications out of the aquarium. It’s for that very reason that you need to remove the chemical filtration media and turn off your protein skimmer when you are medicating an aquarium. I would think filtering your aquarium with either a good brand of activated carbon or a set of polyfilter pads should be sufficient if you want to add some mushrooms to your seahorse setup, Sean.
And if you want to add some more color to your seahorse setup, you may also want to consider some of the very lifelike synthetic corals, gorgonians, and sponges offered by Living Color (http://www.livingcolor.com/coral.cfm). They come in a variety of colors, are extremely realistic, and have an excellent texture, all of which makes them ideal for use in a seahorse tank.
Best of luck with your new seahorse system, Sean!