If you plant a Dust Cap in a new Farming Tile or Planter Box, the Dust Cap needs a delivery of Slime to grow, However, if you uproot Mealwood from a Farming Tile or Planter Box and then plant a Dust Cap, the Dust Cap tile needs a delivery of both Dirt and Slime to grow.
Steps to Reproduce
Create a Farming Tile or Planter Box. Plant Mealwood in it. Uproot the Mealwood. Plant a Dust Cap. Examine the Farming Tile or Planter Box immediately after the Fungal Spore is delivered and it will show Awaiting Delivery Dirt and Awaiting Delivery Slime.
This issue has not been confirmed by a developer yet.