The Warped & Crimson Nylium are blocks that are found in the Warped & Crimson Forests. They grow in the Nether similarly to how grass blocks grow in the Overworld, and it will grow Nether Roots and Fungi on top of it. These blocks come in red (Crimson Nylium) and blue-green (Warped Nylium) variants.
You will need a silk touch pickaxe to obtain it, otherwise it will drop netherrack.
You also can obtain it from chests in bastion remnants with hoglin stable chests.