Algae needs light and nutrients to grow.
Do larger/more frequent water changes, don't overfeed the fish, and lower the hours the light is on.
By doing water changes, you are diluting some of the nutrients in the water. By cutting back on fish food, you are a) leaving less decaying food to be used as algae food and b) the fish will produce less waste, which is again algae food. Lowering the light gives algae less fuel to reproduce.
Do you have any live plants in the tank? If not, a complete blackout for 5 days usually kills the algae. Then just make sure you do not 'feed' the algae again to allow it to grow.
If you just want to clean the aquarium deco items, soak them overnight in a solution of 1 part bleach, 3 parts water. A light rubbing with a cloth or old toothbrush will usually clean the rest off. Be sure to RINSE and RINSE and RINSE the deco items and then let them dry completely before returning them to the aquarium.
