when a building reaches level five, and I click on it, it shows an x to close or a coin. If I click..

A green coin icon means you have a coin you may collect. Gray icon, means not ready yet. All 3x3 buildings produce coins at the rate of 1 every 40 minutes, until it reach max of 15 in 10 hours. But the coin notification does not appear above it until 5 coins have accrued. If you tap on it before 5 coins have accrued, you'll get the green gear icon and can collect the few that are ready.

Similarly, the 4x4 Estate produces coins, but the rate is 1 every 10 minutes, with max of 60 after 10 hours. The coin notification will only appear when 19 coins have accrued.

BTW, it has nothing to do with level 5 of the building. Buildings will produce coins after level 1 is complete.