X2
Separating him is a good idea.
As @MrsBrooke has suggested, it could be from a peck or debris in the eye. MG is also associated with bubbly eye as well.
Flush the eye with saline and check it over well, look for any debris in the eye. If there is anything in it, try to remove it. Monitor him for facial swelling, mucous and signs of respiratory illness.
Having a flock with MG it's not necessarily a "death sentence", but it depends on your goals.Do you sell/give away hatching eggs, chicks, started pullets, etc.?
http://edis.ifas.ufl.edu/ps044