You get hair algae if your params are way off.you can have a small episode if you bring Inna coral that has some on it..but as long as your params in your tank are good , The tangs usually take care of them and won't be an issue
There are so many times, where I've placed a frag in my tank, which brought, bubble, hair, flat worms and one time, ich. This is a good point, the only way to avoid is to set up a QT for the frag. OR the frag is removed from the base. The base plug is the problem. For me, I simply remove the frag from the plug or use only fresh cut frags, then mount them on my own plug, which I place into my main tank. QT for a frag is probably not needed at this point, except for parasites that live on the corals itself.