Algae Smell
If there's a smell of algae in your bathroom plumbing, you'll notice it at your sink and shower drains. The musty smell fades when you run water but returns soon after you stop the water. The problem usually occurs in drains that tend to clog. It is most common in bathrooms that are poorly ventilated and without fans.
Cause
The cause of the algae smell is algae or mildew growing inside your bathroom drains. Both growths thrive in damp environments. A pipe with organic build-up inside it helps to trap water for algae, as well as providing a food source for it. Not only is the smell unpleasant, but exposure to mildew spores can trigger allergic reactions in some people, including symptoms like headaches and respiratory problems.
Solutions
To get rid of the growth inside your plumbing and the algae smell, put 1 cup baking soda down the drain. Let it sit for 15 minutes, then pour 1 cup hydrogen peroxide down the drain. Hydrogen peroxide is an oxidizer. It destroys organic matter like algae and mold. Baking soda deodorizes the pipe. Give the baking soda and hydrogen peroxide about an hour to work, then heat a teakettle full of water on the stove. As soon as the water boils, pour it down the bathroom drain. It washes away any lingering organic matter left inside the pipe.
Considerations
You can make your bathroom less attractive to algae and fungus. That will keep the musty smell from returning to the plumbing. Install an exhaust fan and run it when the shower is in use. Opening a window reduces humidity levels in the room. Once a month, pour 1 cup of hydrogen peroxide down problem drains. It will clear away any spores inside the drain before they can get established and start to release that unpleasant algae smell.