After your tank has cycled and the inhabitants seem happy and healthy, after some time you’ll see that your tank glass will develop a covering of algae and some of your more delicate corals may not extend their polyps as much or perhaps at all.
Assuming you haven’t dumped a bottle of Windex into your tank, it’s likely that you have excess nutrients and need to identify what exactly is going on and fix the issue(s).
As a start, I would recommend you test the concentrations of Phosphate and Nitrate in the water. The suggested level for Phosphate is between 0.02 and 0.05 PPM and for Nitrate a level of 4 to 8 PPM. Any levels higher than these should be addressed. For Phosphate testing, I recommend the Hanna Instruments HI736 Phosphorus Ultra Low Range Checker HC for Saltwater Aquariums.
To get the phosphates down, you need to watch the feeding to keep it down, do water changes and potentially add a granulated ferrous oxide (GFO) reactor to your system. There are several reactors on the market. But to keep the cost down, I would recommend Two Little Fishies ATLPBR550 GFO PhosBan Reactor 550. Additionally for using the reactor, you will need to purchase the GFO and a water pump to circulate your tank water.