Ok, just read through the posts....
He/she doesn't know chicken feed, out in the "wild" they scrounge for what they can get .... Papaya & bugs. Try wetting or mixing the feed with the papaya but remember the more fruit it eats will be softer poop. Then decrease the papaya each day.
You have a Dept of Ag, call them to see if they'll do testing got the poop for you. You also need to provide some grit, try putting some ACV (with the Mother, unfiltered, Bragg) in it's water (1T/gal).
It's in a strange new surrounding & scared. It could be that you were able to "catch it" due to it may have been enjoying the sun or just weak from not enough protein, although think Kauai's been getting alot of rain lately? It's going to be awhile for it to "trust" you but with the cats around, that maybe a problem.
Keep an eye on his poop & crop. Watch the color of it's eyes, from what
@Ravynscroft (correct me if I'm wrong) has mentioned it turns "grayish"? Haven't heard of Marek's in the Islands, more issues with Dry Pox due to the mosquitos.
Edited - Can you take a pix of it's poop? You are familiar with cecal poo? Thinking your chicken is about 8 - 12wks, maybe be a pullet?