Since you do have a younger bird/birds it would be wise to treat with Corid. Chickens do develop resistance as they get older but they are never totally immune, older birds can and do come down with coccidiosis if they are exposed to new strains such as when new birds or young birds are brought in, or if their immunity is compromised for some reason.
And no, it never does any harm to run a course of Corid. All Corid (amprolium) does is retard the growth of the cocci protozoa by starving it, it doesn't kill it all off. In this way the bird is protected from an overgrowth and has time to develop resistance to the strains of cocci in it's environment.
As a side note, do not mix ACV with the medicated water.
It's possible this could be shed intestinal lining but I'd not want to risk not treating them since this is not the first time very recently that you've seen this. I very, very rarely see shed intestinal lining. It's just always better to be safe then sorry if you even suspect there's a chance of coccidiosis. No reason to risk loosing birds. If this continues after treatment then you'll have at least ruled it out as a cause and can move on to other possibilities.