Algae is something that will quickly come to your attention mid-way through the game. You find yourself needing to create something like fertilizer and with this, you will need three units of Algae and more than likely have not come across any of this yet. Here is where to find Algae in Planet Crafter.
In order to get to the point where you can even obtain Algae, you will need to build an Algae Generator item in a body of water. This of course means you will need to have progressed far enough in the terraforming process for there to be lakes on the planet.
Once you have left an Algae Generator in the water for long enough, you will begin to see that plants start to grow in the water. It may take some time before they begin to appear but if you swim beneath the water and look up at the plants, you will see that there are lumps of algae that you can harvest.
When you get close enough to the Algae, the game will highlight it, as you see with pieces or ore and other items in the game. You can simply click on this then and it will add the algae to your inventory.
Every few minutes, you can come back to the body of water and you will find that there is more Algae ready for you to harvest. If you are tight for time, you can start crafting more Algae Generators. This will be good to go toward the terraforming process but it will also result in considerably more algae forming in the water for you to pickup.