You can freeze it but it will change consistency when it's thawed and become kind of grainy. I experimented when I had to go out of town and still had part of one of those big containers. You can try it and see if you don't mind. If you use it for something like smoothies it's probably not a problem.
I am surprised you saw the mold so fast. I have never had mold happen to me with live yogurt even though I regularly keep yogurt in the fridge (unopened) way past its expiration date. Like, there is some in my fridge right now that I bought 6 weeks ago that I am working through and it's fine. (Long story involving a premium yogurt brand that I only buy when it's "marked for immediate sale".)* If it's "live" yogurt, normally the only thing that should happen is to get a little more yogurty (sour), because the yogurt bacteria transform the milk so that other bad critters can't grow there. But perhaps the wrong spore just happened to land on the top of that container after you opened it. I am always careful to go in only with a clean unused spoon, and I do use it within 3-4 days after I open it.
*But you might not want to live as dangerously as me. "Posts by members, moderators and admins are not considered medical advice"