Algaes
I dont know if adding fish will be your cure.
Algaes thrive on a lot of elements. If it was a bloom I would look into your feeding or dosing. Make sure your rock stays clean and also keep up on proper water changes. A lot of people do 10% but sometimes a specific tank will need more. Loke mine I do 20% only because I feed a little on the heavy side.
Fish will only temporarily hide the problem...You need to get to the root of the problem meaning what is fueling the algae to grow. May just be a typical cycle all tank's go through, but we'll need more info as already suggested to determine that