Typical new tank appearance. An algae sponge or scrubber will be in order. As for surface, Get at least 6-8 clean up crew (CUC) such as turbo snails, margarita, astrea and nerite snails for clean up.
If you haven't yet done so, get a good water test kit which for at minimum. . .. . Ammonia, nitrite, nitrate and PH of water. Additionally, Fish appears to b e thriving but was placed prematurely. As others have indicated, take one step back as there is Nothing that happens fast in saltwater and unlike freshwater, you must go slowly, feed slowly, stock slowly, etc.
hexagons are by no means ideal but can work if maintained properly and again going SLOWLY.
Lighting will be important as is water flow for sustaining sea life. A 20 hex has limited space but you must focus on an LED strip with High blue(s) and not have the tank very white in brightness. A nano powerhead such as Koralia nano will be essential for water flow and may even reduce the algae a little. Also consider an HOB ( hang on back ) all
in one with a skimmer or a backpak unit for reduced maintenance and effective biological upkeep of tank. Do not let this overwhelm you as you can do this in steps as is affordable. This is a whole new world especially to those in freshwater but is also rewarding.
Last but not least...………..
