It doesn't eat them til they die, they use the nutrients to get strong enough to reproduce. They voluntarily fall off of the fish in order to reproduce. One ich organism jumps off of their host to create 200+ more just like them, considering your fish was peppered with them, multiply those by hundreds, than those by hundreds etc. It needs to be taken care of. FWD do nothing to the cysts because they are on the body with a protective coating. When they leave the body they fall and create a protective coating, the only time they are vulnerable is the short time they are swimming to find another host. The second part to FWD is that if you don't have temp and PH just as it is in the tank, you can kill the fish.
Their cycle is why there is so many myths about treating ich. If you took what you did today as a cure, than there will be threads all over saying the new way to cure ich is to chase them with a net under a rock in 10 minutes they are CURED!! As silly as that sounds the same happens everyday. Hobbyist get so frustrated with ich that they try anything. Add a little garlic to the system, (at the same time the ich that is on the fish go through a cycle and fall off.) the hobbyist sees that the fish have no visible spots, garlic becomes a cure. Only way is to net ALL fish, put them into a seperate tank, and a copper or Hypo for 6-8 weeks, there is a lot more detail that is covered on those threads about treatment. At the same time your display tank is being cured by not having any fish host. NOTE: only fish get ich, anything else can stay in the tank. After 6-8 weeks of treatment. It should be completely ich free 99.5% guarantee.
Anything wet going into the tank after treatment needs to stay in the QT for atleast a month, Inverts, rock, sand. even water that has been in a system with fish. Not treating it with hypo or copper, just separate unless it's a fish and it shows signs of ich. At that point you treat counting the 6-8 week period from the day you notice the spots are gone.