First off did you ever see a spike in ammonia or nitrite? if you are saying your ammonia is 0, nitrite is 0 and nitrate is between 0-20 you can add a clean up crew of snails, hermits and crabs. I would then wait another couple days to make sure parameters stay good then add your first fish. What type of lights do you have? If they are coral capable you can add corals around the time you add fish. Also, I highly doubt your coralline is spreading it takes months for the stuff to start growing. I would make sure it isn't Cyanobacteria as it can look similar to coralline but peels off easily and is a pest algae.
20 Gallon SPS reef
29 Gallon FW "fat catfish" tank
20 Gallon Long Dart Frogs "Zig" and "Zag"