I have found a Mooshroom Island! However it is absolutely massive, and I want to convert (at least most of it) to grass.
What is the most effective/efficient way to do so?
There are several methods that can be used in order to do this.
The most efficient (and most commonly used) method would be to cover the entire island in water, wait for the Mycelium to die, the use sponges to clear the water and seed the dirt with Grass blocks. This uses minimal resources, and requires minimal effort to achieve.
There are however other methods that are reasonably simple and cheap to achieve the same.
1. Brown mushrooms (Bedrock Only)[requires confirmation]
On Mycelium you can grow mushrooms to large mushrooms at any time of day. If you plant brown mushrooms in a grid, 6 blocks apart, this will cover all the Mycelium from the sun, bar a 2x2 square (the corners of the brown mushrooms are missing). You can then use these spaces to seed the Grass. You can watch a full tutorial here
Normally, Mycelium cannot be tilled. However, it can be converted to a path block with a shovel, and then the path block can be tilled. Without a nearby water source, the tilled soil with then gradually convert back to dirt blocks. Additionally jumping on the tilled soil will also "break" the tilled soil, and convert it to a dirt block.
Mycelium cannot be tilled with a hoe to make farmland.
However, it can first be converted to dirt path using a shovel, then tilled with a hoe to create farmland.
3. Cover it up
This is by far the most taxing in both time and resources. By placing another block on top of the Mycelium (e.g Sand or cobble) this will eventually kill off the Mycelium, allowing you to seed it with grass blocks. This can also be done in chunks, by placing borders around the areas of dirt to reduce the spread of the Mycelium onto the dirt. Reducing the Simulation Distance can also help to control this.
Or, ultimately you can simply cover the entire area with Grass blocks. But covering an entire continent with Grass blocks would take forever.