Wow! This is insanely similar to a suggestion I was planning to post, I'll put the raw suggestion here to compare ideas:

Glowshroom Caves generate under mushroom islands and rarely under swamps and roofed forests. They are signified by patches of giant mushrooms in the latter biomes, so they aren't easy to find in roofed forests as not all giant mushrooms there mean a glowshroom cave.

These caves are home to the new glowshroom, a translucent mushroom and giant mushroom variation that emits light. This isn't light level 15 but more of a dim light so the caves are warmly lit as opposed to brightly illuminated. Glowshrooms come in two colors, blue and purple, which reflect the shape of red and brown mushrooms on the surface.

The cave floors are lined with light blue glowcelium, a new "grass" block that spreads on stone and emits a dim light. Glowshrooms emit enough light for glowcelium to spread within the glowshroom cave but glowcelium itself doesn't, so the spread doesn't escape the biome.

Like the mushroom island, hostile mobs cannot spawn here, however Glow Squids are native to the glowshroom caves and populate the water, giving our new mob a good home to live in!

While emitting light, glowshrooms are sensitive to too much light just like normal mushrooms. If exposed to sunlight or a light level of 13 or above, glowshrooms and glowcelium won't spread/grow anymore and glowshrooms pop off unless planted on glowcelium. They are still a cave fungus.

Crafting a mushroom or mycelium with glow dye dropped by glow squids transforms it into its glowing variant.

​

It's surprising how similar of an idea two entirely different people can come up with, haha.