I accidentally removed Poseidon, how can I get it back?

Check your storage ship. If it's there, drag it back on the island and your flowers will return.

If you sold it, you cannot get it back. You'll have to play often to earn coins to buy something else with high flower value instead. Look also in the season set for high-flower value decoration.

Since it isn't possible to sell Poseidon (as it was a gift from Wooga for achieving some landmark) you must have put it into the storage ship by mistake.