Liquid poop!!

Should I be concerned if my cat has had a completely watery loose stool? He is acting and eating normally. 

You should be concerned but not overly worried. Have you been to the vet about it?

It's only happened the one time so I'll keep an eye on him.
I also noticed that another one of our cats, Bella had worms in her stool so we have just given them all worming paste (exelpet - will never buy it again!)except BB (the one who had liquid poop) who we couldn't get to take it no matter what we did.

Sounds like a good idea. One runny poop does not an emergency make. Lol Hopefully this was a onetime thing.

IF BB continues with the liquid diarrhea, check for worms from him too.  You might need to figure out how to get that dewormer into him too, somehow. 

We might have to buy a new one. That stuff is horrible!