Unfortunately your monti is looking a little worse for wear
This looks like it's bleaching all around which is a bad sign for sps corals but it's reversible with some TLC, luckily it doesn't look like tissue recession though because if it were there's not a lot you can do to save a whole coral from it but frag the parts that aren't.
What are you parameters? More specifically what's your salinity, Alk, Ca and Mg? Phos would help as well but monti's are pretty tough so we'll see if we can find the problem. Do you have any other sps in your tank and how are they looking? Any similar signs and how are the rest of your corals looking as well?