type of edible mushroom
蘑 refers to a specific type of edible mushroom. Its meaning is narrow and concrete, centered on this particular fungus.
蘑 combines the grass/plant radical 艹 with 磨, which likely provides the sound. The character is used for a type of mushroom, a plant-like fungus.
The top grass radical 艹 suggests a plant, and the bottom 磨 (polish, grind) can be imagined as the mushroom's cap being polished smooth. Together they point to a specific edible mushroom.
For マ, picture a mother (ママ, mama) picking this mushroom in the forest: mama -> マ, and she knows exactly which one is edible.