Green coins are the currency you get for completing a level. To complete a level, you need to accumulate the specified amount of coins in any way. The easiest way to accumulate money is by completing orders for the restaurant.
When you pay the starting debt, a message will appear "the restaurant is now open". At the top right there is a button that opens the restaurant showcase, there you need to select the dishes you want to cook. When there is more than one dish on the showcase, a customer can swim to you, he will have an order.