It could be "ich"( crytocaryon irritans) or it could be flukes which seem to be showing up a lot lately.Or it could be both or niether. Confused yet. It's almost always difficult to figure out what's wrong with a fish and then you need to choose the right treatment and worry about the other fish catching whatever it is.
I don't think I've seen or heard of a seahorse catching" ich" but my experience with them is very limited. I do believe they are susceptable to flukes, though.
Take a look at this thread . The symptom descriptions and pictures may help you.
http://reefcentral.com/forums/showthread.php?s=&postid=11276835#post11276835
If it were my fish.
And if I concluded it was flukes. Iwould treat it with prazi pro. I've used this product and it works. If you can't get prazipro quickly you can do a fresh water dip for 3-5minutes or better a bath in formalin at the reccomended dose( depends on which formalin product you use,. some are 3% others 37%) for 50 to 60 minutes. The formalin bath should also kill any fluke eggs on the fish. The freshwater dip will not. The prazipro is milder and less stressful but if your carefull with the formalin or fresh water option ,the fish will tolerate it well.
If it is cryptocaryon irritans "ich" you can try hyposalinity. Drop the salinity in the quarantine tank to 1.09sg ( you have to be very accurate with this and nedd a refractometer to do it)for several weeks and then raise it back up slowly over the course of a week. Many have success with this method. I tried , the ich went away but when I raise the salinity back up some eggs musta have hatched and it came back but appeared weak (hagin on the fish but not really penetrating. At this point I treated with copper sulfate, This is the other option and it will kill the parasite with a two week course of treatment.
The live rock you have placed in quarantine should not be reintoduced to your main tank. For more denitrification if you need more you can place a small tray of crushed coral or sand from lyour tank on the bottom of the qt tank. Check the ammonia carefully and frequently. There is a device that sells for a few dollars called an ammonia alert badge wihich is usefulll.You do not ahve to worry about nitrite or nitrate in a fish only quarantine tank. Nitrite is exponentially less toxic to marine fish than it is to freshwater fish due to the abundance of chlorides in salt water which neutralize ist's toxicicty greatly.
Godd Luck
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