Maybe he's trying to cough up a hairball ..
You can get hairball medicine at the pet store..
The liquid is just mucus that helps to bring up the hairball.. it's usually not much. It happens long after he eats because he spends time grooming after meals.

Give it your flesh to eat .

Try changing his food and the timing. It may have nothing to do with timing, but you need to figure out what the cause is. People that don't spend time and know nothing about his surroundings won't be able to pinpoint anything, you have to manipulate his surroundings until you figure it out.

I adopted a stray when I was a kid, but don't have one as a pet yet. I do feed strays though, this one is the only one that will let me get close to him

My dog does this if the gap between dinner and breakfast is too long. I moved breakfast time up a half hour and she's been good since.

Hair ball? Check we with your vet

Most cats throw up. It's normal. Carefully examine what is in the sick.
They swallow their own hair when licking and grooming themselves. It's not digestible, so they chew a bit of grass to help them vomit up the hair.
It's possible to reduce this to almost nil by grooming the cat every day. They absolutely love it.
(But if you have a long-haired type and if that hair has become very matted, it would be far kinder to get the cat sedated and shaved at the vets. Then start the grooming from the first day home and keep him/her warm while the hair regrows.

Don't overfeed. A cat's stomach is only the size of a golf ball and if they eat too much they are very likely to sick it up. Better to feed two or three fresh meals per day, and remove whatever the cat doesn't eat. (I give leftovers to the birds, dusted in DCP (dicalcium phosphate).)

If you don't see hair in the vomit, then the cat may have a food allergy. Most adult cats are allergic to milk, and even the commercial "cat milks" may still cause vomiting. Kittens need special milk - never cow's or goat's.
If milk is not the problem, then you may need to do an elimination diet to find out what the cat is reacting to. Lamb is often one of the most easily tolerated foods.
A home-made food is always far healthier than any of the manufactured brands - but it's costly and time consuming. I make a month's worth at a time and freeze them in golf-ball sized portions. One can look up the ideal natural cat diet online - or you can message me and I'll send you the recipe I found which worked very well. The most important thing to add is taurine; cats need it for good eyesight.

Ask doctor or google...

It could be a hairball.

U definitely need to see a vet immediately do not wait !!!