Trixie is trying to take control of your cauldron! To stop her, you need to match Candy on the board to create Pink Magic Squares. Trixie will create Green Magic Squares to counter your pink ones. You can see who is winning in the battle by looking at the Pink and Green bars that surround the cauldron.
You can make matches over Trixie's Green Magic Squares to convert them to Pink ones too.
If you have more Pink Magic Squares than Trixie's Green ones you will defeat her and your cauldron will be safe...for now!