I can think of three possibilities right off.
First, the seahorse MIGHT possibly feel stressed from the firefish addition.
Second, it could be affected by parasites that are getting to plague proportions. They could be ones already present on the seahorses and now stress levels allow parasitic expansion, or, it can be parasites introduced to the tank via the firefish addition, ones the seahorse has NOT grown up with. Seahorses DO NOT DO WELL when introduced to new types of parasites.
Lastly, the water quality could be deteriorating gradually and there might be bacterial problems beginning either internally or externally. THERE ARE NO TEST KITS that will tell you when this aspect of the water is degrading to the point bacteria are expanding. For that reason, we need to be diligent in our protocols for husbandry and water changes.
At this point, what you have posted isn't enough information to be able to recommend a solution, but I personally would be getting prepared by getting a hospital tank (10g preferably) ready and having Furan II and/or tri-sulpha antibiotics on hand in case it ends up being bacterial.
If it starts doing scratching or rubbing against decor then I'd do a 12 minute fresh water dip, matching temp, s.g., and pH and watch closely for initial reaction. If parasites are problematic you will see thrashing about of the seahorse at first, gradually becoming calm. You will see minute particles that seem to be exploding from the seahorse and these are the parasites. Leave the seahorse for 12 minutes and only remove it if it becomes unresponsive to touching.
Perhaps the simplest move is to remove the firefish to another container for a week or so and see if the behaviour of the seahorse changes back to normal.