Yeah, unfortunately that's kinda normal with new wild colonies.
It doesn't matter if they are bigger or smaller colonies.
When you quarantine you avoid spread the infection to the display system, but that can still happen after a long time, in a mature system, sometimes years after!! Even when you dip/quarantine the new polyps. Just a drop of water could transfer the pathogen, of course. The pathogen doesn't aways manifest itself staying "dormant" in the system if it can find it's way in. Something in the system trigs it to act. Normally it's a damaged on external tissue and/or poor water quality. Other things will help the pathogen, of course. As aways: many variables...
That said, there is almost no way to securely eradicate 100% any type of zoa pathogen from our systems. We just need to be alert. Some of them sometimes won't be introduced by pure luck.
It's easier to eradicate Aiptasia anemones then those type of infection pathogens, when introduced!
Trust me on that!!
I've had systems with colonies totally treated (dips/quarantine) and virtually sterile/healthy and after years, without any new new introduction, they began to show signs of infection and get rotten. So sad!!
There are many types of infections, by the way...
The one you're talking about comes with wild colonies and normally is much more active right after shipping stress for some reason. It covers the polyps with a whitish "web" or film, like a fungus. The polyps smell really, really rotten!!!
So what to do?
Tips:
Dip all new zoas.
Quarantine all new colonies (wild or not!!) for as long as you can.
It doesn't matter the size of the colonies/frags.
Quarantine makes much easier to treat!
Keep all parameters stable and good quality light/appropriate flow.
Maintenance schedule with regular partial water changes and tests to check the system.
And... good luck!
Grandis.