I suspect it's an algae bloom like diatoms. You are experiencing the uglier post cycle algae and bacteria bloom phase. It can get ugly, and it should peak and almost go away over the next 2 to 4 weeks.
My advise is, don't use it unless the situation gets REALLY ugly.
This is a bloom. It will use up the available food and then die off. If you artificially keep the bloom from happening, the food will still be there and the bloom will happen when you quit using that treatment. That stuff is meant for use with long term algae issues, not start up blooms.
Oh you are going to be seeing LOTS of different things growing in there over the next 12 months. First film stuff, then brown stuff, then green stuff, then a different green stuff, then a different brown stuff, etc etc.
For 6 months, just manage your salinity and temperature and perfect your water change routine. Don't let all the ugly phases get to you. Accept the fact that you will have ugly phases. Post a pic now and then to learn what phase you are in but don't intervene. Your biome is just going through the natural maturation it needs. There are many organisms each battling for their place.
Invest in some test kits over the next few months -- no rush.
Nitrate - I prefer NYOS
Phosphate - really it is just Hanna ULR Phosphate (or Phosphorus if you want to count in ppb)