something is killing the monti, and letting algae grow on the the skeleton.
could be many causes. usually montis dont need that high of lighting, so cant be it, what is your lighting ?
I have seen this happen to monties who get too much flow as well. check that no direct power head on it.
lastly, check your DKH, if sudden jump, it may cause burned tips, which will allow algae to grow on dead parts.
with good water quality, and water movement and lighting, it should come back in no time, but do check and test all parameters to find the cause, salinity, temp, KH, CA, MG. ...
JME,