Minecraft wiki entry for Nether_survival, associated text: They need space to grow.
After it is grown into a huge mushroom, the nylium below it decays into netherrack, so you need to put bone meal on it, which turns the netherrack back into nylium if there is nylium adjacent to it.
The blocks are fastest broken with an axe. They can be hard to reach from the ground, so you can build stairs up to a platform that lets you jump on top of the mushroom.