The floss prefilters are like your filter cartridge in your whisper power filter. They become nitrate traps when you leave them in there for a long time while you are cycling I do not think they will be a problem. When you have fish and feed them some food get sucked up your filter and just stays on it and your clean up crew cannot get it, thus the build up of nitrates. You could have left the shrimp out since you put your live rock in, I would think a couple of days would be enough though. Test for ammonia every other day and you will notice it will spike and then come back down.