Mucusy diarrhea?

I just went into the bathroom, where our cats litter box is, and there was liquidy stool on the bath mat, a spot a little bigger than a quarter of bloody mucus near the toilet, and some bloody, mucusy liquid stool in the litter box. What could be the sudden cause of this? I'm assuming that it's Gus, our 5 month old kitten, based on the size of the piles and that Ramsay has hardly been in the bathroom since I got home. Neither of them have ever gone outside of the box. He hasn't eaten anything out of the ordinary, just a couple bites of boiled chicken and his usual dry kibble today. Should I be concerned about the blood/mucus?

I'm going to go wash the rugs and disinfect the litter box as well.

Also, and I know I'm probably just being totally paranoid [we lost a 18 month old cat in January totally unexpectedly so I get paranoid lately] but we just moved into an apartment last week where the lady living here previously had 2 cats that never saw a vet.

when a cat has blood in the poops its never good. u should take the cat to the vet as soon as u can. i dont know what could cause it. there are many reasons but it needs to be treated now. keep us updated and lots of healing vibes for your kitty.

There was no loose stool in the box this morning. If he just ate something that he shouldn't have would it have caused the blood/mucus?

im not sure but its better to at least call a vet and see what they have to say. 5 years ago my dog had bloody stool. we took him to the vet and he ate a plastic frisbee. it cut the inside of his stomach and caused all the blood. if not treated if could of killed him.
just give your vet a call to be on the safe side.

Those are colitis type symptoms, and yes, it could be caused by "a dietary indiscretion" according to my home veterinary book. My cat had similar symptoms once when I fed him a new food, and they cleared up when I stopped feeding it. If his poops look normal again, and he's eating, drinking, acting fine, etc. I wouldn't worry- but that's just me. It wouldn't hurt to call your vet if you're worried.