I wouldn't waste time on specific plastic plant cleaners. They sound like a gimmick and waste of money.
I just use normal household bleach (sodium hypochlorite). Get a bucket or some sort of plastic container with a lid. Pour some household bleach in, either straight or mixed with water. Put the plastic plants in and put the lid on it. Put the container outside so you don't have to deal with the fumes. Leave the plants in the bleach for 30 minutes or more if need be. Any algae will be broken down by the bleach.
Then remove the plants from the bleach and rinse them under tap water. After rinsing, put them in a bucket of tap water and add a triple dose of dechlorinator. Stir them up and leave for a bit. Then rinse the plants again and see how they smell. If they smell of bleach, rinse again and put out in the sun for a few hours. Then rinse and check the smell.
When they no longer smell of bleach, put them back in the tank.
You can also use granulated swimming pool chlorine (calcium hypochlorite) and it does the same thing. It is simply granulated chlorine vs household bleach that is liquid chlorine.
*CAUTION*
Always wear rubber gloves and safety glasses when handling bleach or chlorine. These substances can do a lot of damage to your skin, eyes and lungs if you inhale the fumes.
Always use bleach or chlorine outdoors, or in extremely well ventilated areas. Never use them in confined spaces without really good airflow. If you have to use them in confined spaces, wear a respirator/ gas mask so you don't inhale the fumes.