Need to identify where exactly the play is, could be between crank arm and bb spindle, bb and the eccentric insert or the eccentric insert and the shell.
You'd sort that by tightening the grub screws that are located on the bottom of the shell.
Definitely check the chain tension first though and if you've been riding with it loose then you might want to strip it all down and check that the insert (or the shell) hasn't worn.
Need to identify where exactly the play is, could be between crank arm and bb spindle, bb and the eccentric insert or the eccentric insert and the shell.
Each will need fixed in different ways.