Will a filter sock catch flatworms?

+1, in fact you could argue old saltwater could be actually already stripped of minerals that newly mixed water has, and will have higher amounts of "stuff", such as DOC's, possible parasites, etc.

Newly mixed is the way to go! I have cycled 5 tanks this way, and will continue to do so in the future

 

Water itself isn't "stable". The pH is affected by what's in it (dry rock vs logs), etc. Properly mixed water is fine.

 

lol exactly. Freshly made salt is the nectar and ambrosia of a reef.