Coraline and feather duster questions

[Fishko]

How do feather dusters reproduce? im suprised my dip didnt kill them. I have maybe a 50 throughout my tank. Dont get me wrong i like them, im just surprised my sixline hasnt taken care of them.

Also im having trouble with coraline spreading. Its slowly spreading on frag plugs but not all over my tank. Im thinking my phos and nitrate are too low
Alk 8.0 getting maybe .4 dkh a week consumption. Tanks been up for maybe a year. My lps are all happy, getting full polyp extension.
Calc 440
Mag 1350
Phos .02-.03
Nitrate 1-2ppm almost 3ppm
Sal 1.026

 

[Dragon52]

Don't know how they reproduce. I have 3 & their at least a yr in my tank.