I always thought that algae was what is described below. But I would like to be proved wrong.
I honestly think ,Mold, Mildew, Fungus fit into the same moisture induced areas but I have only seen algae on the surface of water.


Algae (singular alga) are groups of relatively simple living "AQUATIC" organisms that capture light energy through photosynthesis, using it to convert inorganic substances into organic matter.

Mildew refers to certain kinds of mould or fungus. In Old English it meant honeydew (a substance secreted by aphids on leaves, formerly thought to distill from the air like dew), and later came to mean mildew in the modern senses.[1]


Powdery mildew growing on a leaf. The ladybird beetle is a Psyllobora species which feeds on the mildew.The term mildew is often used generically to refer to mould growth. Moulds can thrive on any organic matter, including clothing, leather, paper, and the ceilings, walls and floors of homes with moisture management problems. Moulds often live on shower walls, windowsills, and other places where moisture levels are high. There are many species of moulds. In unaired places, such as basements, they can produce a strong musty odor.

What most horticulturalists and gardeners call mildew is more precisely called powdery mildew. It is caused by many different species of fungi in the order Erysiphales. Most species are specific to a narrow range of hosts, and all are obligate parasites of flowering plants. The species that affects roses is Sphaerotheca pannosa var. rosa.
Another plant-associated type of mildew is downy mildew. Downy mildews are caused by fungus-like organisms in the family Peronosporaceae (Oomycota). They are obligate plant pathogens, and the many species are each parasitic on a narrow range of hosts. In agriculture, downy mildews are a particular problem for growers of potatoes, grapes, tobacco and cucurb