Sounds like a diatom bloom. I'm guessing by the sounds of it that the tank hasn't been up for long. Is that correct?
Diatoms are common in new tanks and should die-off soon enough. no worries. You can however check your PO4
levels to ensure that you don't have a phosphate problem that will further help the diatoms get a foot hold. If your PO4
is good, and you aren't doing anything to add them, they shouldn't last too long.
As far as adding calcium. If your tank is in fact new, just let the tank settle a little before adding anything.
Also, your lighting schedule is overkill for a new tank. That will also foster the diatom bloom. Cut it back to 6 hrs or so, until the bloom is gone then gradually increase it.
Is your new LR