Yes, it sounds like your tank is experiencing a bloom of diatoms. As I have are ready pointed out, they are harmless and most newly set up marine aquariums go through a stage where the diatoms or brown algae grows on surfaces in the aquarium. In most cases, the brown algae will disappear as suddenly as it appeared once it uses up the available supply of some key nutrient in the aquarium (usually silicates). Ordinarily, once the available silica has been exhausted, the population of the diatoms will crash and they will then typically die off on their own.
Otherwise, we have been discussing some of the simple measures you can take to help eliminate the diatoms in this thread. As I said, brown diatom algae is usually the first problem algae that a new marine aquarist encounters. A bloom of brown algae often occurs soon after one introduces new live rock to a marine aquarium. This bloom occurs because the curing of the live rock introduces silicates and nutrients (even pre-cured live rock from your LFS will have some die off after it is transferred to a new aquarium; that’s normal). As a result of the diatom bloom, a brown film soon coats everything inside the tank.
Control of brown diatom algae is relatively easy. The first thing to do is to purchase Trochus or Astraea snails that eagerly consume the brown diatom film. I’ve had good results purchasing Trochus snails from IndoPacific Sea Farms (IPSF). There are other snails that will clean the glass such as Nerite and Strombus snails, but Trochus and Astraea snails are the brown diatom cleaner workhorses. The second thing to do is to perform regular water changes to remove any excess nutrients and silicates from the water. The third thing to do is to have an effective protein skimmer to help with the nutrient removal. The fourth thing to do is to cut down light intensity or duration. The final thing to do is to have some type of chemical filtration such as carbon or ChemiPure help with the nutrient removal. I would rank the methods above from most important to least important in the order they are listed.
It sounds like you have already implemented several of those control measures. The denitrator will do an excellent job of keeping the nitrates that fuel algae growth under control. And the protein skimmer will help remove dissolved organics before they enter the nitrogen cycle, so that’s going to help. Plus the activated carbon and Purigen should provide efficient chemical filtration, which will also be beneficial in the long run. The best things you can do right now to help hasten the demise of the diatoms are to load up on some of the Trochus and Astraea snails that love to feed on the diatoms and reduce your photoperiod considerably. If you have been running your PC lighting 12-14 hours a day, I would cut that at least in half. Keep your lights on only 6-7 hours a day or less until the diatom population crashes. The beautiful coralline algae thrives best under low light levels and will grow faster at the reduced light levels, whereas the light-dependent diatoms will be adversely affected by the shortened photoperiod.
One other thing to keep in mind is to double check the type of activated carbon you are using. Carbon is activated two ways, either with steam or with phosphoric acid. The type of carbon that is activated with phosphoric acid contains phosphates, which can be leached back into the aquarium water and promote the growth of nuisance algae. So you will want to avoid that type of of activated carbon. The carton or box that the activated carbon comes in should be clearly labeled and state specifically that it is "steam activated" or "phosphate free" or something to that effect if it’s a suitable brand for your aquarium.
In short, you needn’t be overly concerned about the diatom bloom and it won’t be harmful for seahorses or any of your other specimens. Just be patient, reduce your photoperiod, add some Trochus and Astraea snails to your cleanup crew, and the situation will eventually resolve itself.
In the meantime, best wishes with all your fishes!