You can use "must" in guesses, predictions, and explanations:

Oh my God, you must be so excited!

There must be a lot of pollen.

A: Where's your phone?

B: Oh, I don't know. It must be in my jacket pocket or something.

In the example at top, the man thinks "It must be clogged." "Clogged" is an adjective. Here are some other examples of "___ must be" followed by an adjective:

It must be expensive!

Your mother must be so proud of you!