In Java Edition 21w16a, some blocks and items can now be placed on Mycelium such as grass, ferns, saplings, flowers, sugar cane, sweet berries, and dead bushes. In Bedrock Edition, however, you can't.
Step to Reproduce
1. Place down a layer of mycelium
2. Attempt to place some of the blocks mentioned in the summary.
---> Doesn't get placed on mycelium.