Normal seeds and sprouts won't work to unlock these clouds. In this case you'll need Grimm Seeds and Sampling.
To obtain Grimm Seeds, you can try one of these:
  1. Tap on Graves
  2. Dragon Egg Chests/Nest Vaults harvested from Grimm Trees
  3. Open Grimm Chests
  4. Spawns from Grimm Trees
Merging these seeds will create the Grimm Saplings needed to unlock the other cloud.