The yellow-brown algae you noticed is most likely due to a bloom of diatoms. 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. So right now, while your tank is still cycling, you needn’t be concerned about the abundant diatoms — in all probability, that problem will soon solve itself.
If not, there are a few simple measures you can take to help eliminate the diatoms. 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 of 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.
However, aside from turning off the aquarium light, you cannot implement any of those measures to control the diatoms until your new aquarium is finished cycling. The snails won’t tolerate the ammonia and nitrite spikes that occur while the tank is cycling, and it’s counterproductive to perform water changes, or operate a protein skimmer, or use chemical filtration media while the new tank is in the process of cycling and establishing the biofiltration. Such measures will actually prolong the cycling process and reduce the numbers of beneficial nitrifying bacteria. So don’t do anything to control the diatoms other than turning off the aquarium light until your tank has completely cycled. Very likely the diatoms will disappear on their own sometime during the cycling process, and no other measures to control them will be necessary.
In short, the growth of the diatoms is nothing to be concerned about and will not pose a threat to seahorses or the copepods and amphipods you have ordered. You can go ahead and see the live rock with the pods when they arrive as usual; they may even feed on the abundant diatoms to a certain extent. A diatom bloom is an easy problem to correct, as described above. If you can update me with your current aquarium parameters (levels of ammonia, nitrite, nitrate, plus the pH, specific gravity, and water temperature of the tank), I would be happy to look them over and determine if any of them need to be adjusted before you order your seahorses.
Whereas the diatoms do not present a serious problem, there are other forms of nuisance algae such as hair algae and cyanobacteria or red slime algae that can be much more troublesome, and you’ll want to guard against them as well. The hair algae is a very unattractive, dark green to black, fuzzy algae that cannot be mistaken for the much larger, desirable macroalgae. Likewise, the cyanobacteria or slime algae grows as slimy sheets that cover the bottom and substrate and are normally reddish, purplish, or black in coloration. Again, you cannot mistake them for any desirable algae. They are entirely different in appearance to the attractive coralline algae that encrust live rock.
So just hang in there for now, arcprolife, and the diatom bloom will probably clear up on its own. If it doesn’t, it will once the aquarium has cycled and you can add your snails and other sanitation engineers, running your protein skimmer, and begin making water changes.