The Slime is a moving obstacle that slips from tile to tile, taking up a few tiles along the way. 
It moves with every tap you make and always goes where the empty tiles are around it.
To solve the Slime, you need to fill tiles correctly next to it to shorten its length.
Keep making matches until the Slime is completely removed from the grid.

Keep in mind that the Slime moves with every move you make, so be strategic in your moves to solve it effectively.