Fungi is the word used for living things belonging to the Kingdom of Fungi, which includes yeasts and molds, as well as the more familiar mushrooms seen below.

Above, the red ones are the famously delicious Caesar's Mushrooms, Amanita jacksonii. Below those are two different species of "coral mushrooms," probaby genus Ramaria. They're not as flavorful as the Ceasar's, but they're OK, much more likely to be found that the better mushrooms, and none of them are toxic, though some species vaguely looking like Ramaria may be mildly toxic.

Blue Mushrooms, Lactarius indigo tastes good but people like to eat it just because of its looks. Raw, it's similar in texture to an apple. Also it can be grilled, used in soups, dried or preserved.

Corn smut

At the right, that a corn smut fungus on a dried-out, infected ear of corn in a cornfield near Tequisquiapan, Querétaro. In the picture, the long, hairy things hanging on the right side are dried corn silks. To the left of the top of the silks the smaller, white items are corn grains. The larger white items with black splotches are mature corn smut fruiting bodies. The blackness is produced by millions of black spores. Before the fruiting body reaches such a mature, spore-producing state, and the body's interior is white and spongy, the fruiting bodies are known as huitlacoche, they're wondrously edible, and in Mexico often appear in soups, with scrambled eggs, or on tacos, as seen below:

Huitlacoche taco with squash flower and grains of fresh corn, or maize; copyright free image courtesy of Cuauhtemoc F. Ramirez A. made available through Wikimedia Commons.

This just scratches the surface of fungus/mushroom-eating in Mexico, though much of the best depends very much on the season, whether it's rained recently, and what kind of ecology surrounds your mercado.