Slime spawning infinitely that causes lag

I and my cousin were just playing normally on a realm when we encountered slimes we were at different spots I saw the slime first that had an oozing effect I believe then I tried to kill it and it splits into smaller slimes that had oozing too then tried to kill those small slimes spawned more of them with oozing again until they created an army of slimes, now on to my cousin he encountered a big slime tried killing it and it turned into smaller ones and tried to kill them with lava but they multiplied now he can’t join cause he keeps on crashing there’s too much slimes

My experiences is the video below and the picture is my cousin’s pov