My cat all of the sudden became very paranoid and seems to be hallucinating. Very hesitant and jumpy when walking across the patio or grass. He is still eating. He is a stray cat we took in 9 months ago. Outside cat.
I doubt he is hallucinating although I am not there, and I do know that sometimes cats see things we don't see, which doesn't mean there is nothing there. Probably your cat had a strange experience on the patio or grass....maybe an insect stung him or there was a strange noise or something, and now he is a bit jumpy about it. I wouldn't be concerned about it unless it goes on for a couple of weeks in which case I might be trying to find out what scared him.
I agree with Mosi. Your cat probably saw or did something that gave him a fright. You should not have to worry too much about the situation unless his behaviour persists for several weeks.
Long ago, my cat had an unexpected interaction with a skunk in our neighbour's yard, and he consequently refused to go outside for a good little while. Cats are just like humans in the sense that they have to process fear/trauma internally before they can resume normal activities with full gumption. My cat is a case in point: he eventually went back outside and was really very happy to do so.
Hang in there! Things are bound to get better for your cat.