Always quarantine everything my friend!!
Just a few of the sps problems that can desimate your captive reef:
Red Bugs
Monti eating nudis
RTN
Brown jelly infection
All of the above are highly contagious, although I believe that a current tank inhabitant must have a compromised immune system to fall prey to either RTN or brown jelly. (the most evil of them all)
Montipora eating flatworms don't seem to have a preference for certain montipora species only. If its a monti, it will be eaten.
I have read of reports where red bugs only seem to flourish in certain acropora species. (something about commencal crabs rings a bell)
Considering the value of specimens in a tank, quarantine is really worth it.
I have my quarantine system hooked up to my display, but whenever I quarantine anything, it goes off line. Once the specimens are to be introduced into the tank, the water from the quarantine is dumped and a new water is prepared in the quarantine before it goes back in line.....
With regards to rtn (what would be my best guess wrt this monti) it will spread if any of your acroporas have a compromised immune system, usually caused by stress.
My advice:
Get that monti outta there, frag it dip it and quarntine it!!!