The term "algae" refers to a class of mainly watery, photosynthetic, and nucleus-bearing organisms that lack the real roots, stems, and leaves of plants as well as their specialized multicellular reproductive structures.
Microalgae have a reasonable rate of growth. Within a day, they might double in size.
Additionally, macroalgae like seaweed are known to develop quite quickly.
Giant kelps have a maximum daily growth rate of 24 inches (61 cm) under optimal circumstances.