Polygamy means "state of marriage to many spouses" and Polyamory is "the practice of, or desire for, intimate relationships with more than one partner, with the knowledge of all partners involved."
The difference between the two is pretty big, since polygamy insinuates marriage while polyamory does not.
Then there's the fact that the genders of the relationships are not specified with the current Polygamy/Polyamory tag. One story might be a usual harem, one guy together with a bunch of girls. An other story might be the same but one girl and a bunch of guys. An other story might have multiple guys and girls in a relationship.
All of these stories goes under the Polygamy/Polyamory, yet they are vastly different. Someone who likes to read about one guy getting it on with a bunch of girls might hate the reverse situation etc.
I therefor suggest that you remove the Polygamy/Polyamory tag and add 3 new ones: Polygyny (One man, more women), Polyandry (One woman, more men) and Polyamory for open relationships.
I know that some people hate when there's too many tags on a story, but I feel like Polygyny, Polyandry and Polyamory caters to so vastly different kinks that it could be warranted to have their own tags.