Nearly microscopic crustaceans that eat film algae. You may spot them with a magnifying glass in your refugium, crawling on the threads of cheato. Feed DTs or Phytofeast [brand name phytoplankton] and provide live rock, sand and cheato in your refugium, then dispense with the filter [which may trap them] and go over solely to live rock and a sandbed. They're small enough to get through a pump impeller, but filter media can trap them and not let them into your tank. So your filter may defeat the purpose of your refugium. Your live rock should be 1 lb per gallon, both for sump and for tank.
When you start spotting them regularly in your refugium, you can get a mandy with fair confidence you can feed him.
If, however, you are replacing filter media, do it a little bit every few days, even resorting to scissors to reduce the amount of media until there is nothing but pure flowthrough. Doing it too fast risks a minicycle.