You need to have some room for the chaeto to grow so if it’s packed in there you should prune.
With regard to the imbalance between N and P, growing just about anything like chaeto or bacteria (carbon dosing) will use up nutrients in proportion to the Redfield ratio of C/N/P (ie a lot more N than P). Also feeding puts those nutrients in your tank in about the same ratio. So you probably have an extra sink of N or extra P from something.
You can get it back into balance by running GFO since it depletes P, or dosing N (stump remover). I personally wouldn’t mess with it unless you are having problems, though GFO is pretty straightforward.