No, PLUS is not f(x)s fanclub name. f(x) Sulli sent out a me2day to Jiyoung saying "plus" i'll see you later and people said that koreans don't use plus so she had to be talking to the fans indicating a fanclub name. She later stated that PLUS was NOT the fanclub name and, even if she wanted it to be SM has not made any official statement.
There is no official fandom name for f(x). It was supposed to be out at Pinocchio era but f(x) and SME weren't happy with it. f(x) fans, i think... is still in the miffle of brainstorming until now. Or probably, f(x) will come out with their fandom name on their comeback this year.