I've had a look into some possible creature designs. Phil and I have spoken that this creature could be from swamps or marshes. One creature that came to mind looking at pictures of micro-animals was the mosquito. Which fits the brief of something horrid and something that could fit the marshy biome that the moss grows. Thumbnail 15 shows three possible stages of maturity based on a diagram of how a wasp pupa becomes an adult, where it seems to 'grow' into the adult shape from your typical grub.
One key thing I might include: This creature is intelligent, and needs to open the door. Because it is much larger than your standard insect, normal insect feet might not provide enough support. So I've looked into perhaps giving it digits to help spread the weight and as a tool for communication.