i have made a custom demension but there live mobs there but i dont want that is there a way i can remove them.
I was searching for this. I found the solution. You can base the dimension off a biome. If you create one or use a current vanilla biome including the nether and the end. You can set what mobs spawn in the biome.
Whenever I make a dimension, I always create a new biome for it, making sure that the biome weight is 0 so it can't be found anywhere else. Though for some reason, custom air and water colors work, but custom grass colors do not.
Make sure you have the color set, if it is set try testing the biome in the overworld. If it works it is probably a bug.