Basically Alex's actor has been accused of sexual harrassment and assault by multiple women. Like so many women that no one can really say "well maybe it's not true!" So obviously that's horrible for all the victims and to a lesser extent it's really annoying to everyone who found him so charming in the show.
Seems to have been doing it from back when he was in an Assassin's Creed game before The Expanse.
People were saying when he did his own behind the scenes videos of The Expanse you could tell the actors playing Holden and Amos didn't like him and tried to stay away (I never watched them to be honest.)