There are plenty of threads in this forum (Predators and pests) about rats and mice. You can use traps, putting them in a weighted shoebox or some similar setup so the chickens can't get to them. Same thing with bromadiolone (Just One Bite, and other brands) which is relatively harmless if the chickens eat the mouse. Which the chickens will, if they see one during daylight.
For adult chickens, the main problem is rats and mice can eat a lot of feed. Of course they can carry diseases in their poop as well. Usually they won't attack the chickens, particularly if there is feed or tasty bait in a trap available.
If you saw one or two mice, you can bet there are plenty more around, and they will multiply. I think I'd make sure that's the animal I saw, though.