Fish Buffet - Plankton - I did not make the required points to move up - what happens?

As Plankton is the lowest level, you cannot be demoted.
You will remain at Plankton level and will receive 10% cashback on your accrued points and restart the level with 0FPs and 7 days to compete.
Example: You earn 500 FPs - you will receive $0.50 in rewards.