Usually with low/no po4 and no3 you will see some things start popping up.....however if it's being processed all correctly I wouldnt think too much of it.
Do you run carbon alone or something like chemipure?
It can be a problem, yes. I’ve been close to zero without problems, but I wouldn’t want to stay at zero. youre starving your corals of an N source with no NH3\NO3. Feed more, skim dry, run fuge light less, etc