Question

What was moving towards the men in the mire? Why did holmes consider this a serious matter

Answer 1

You should have pinned the pictures of your chapter before asking the question, so that we can understand the chapter and answer your question.

Answer 2

Answer:

It's an enormous beast with huge jaws, and it's been covered in some kind of weird glow-in-the-dark stuff. Watson touches the stuff on the dog's fur and realizes that it's phosphorus (a glowing chemical). Holmesapologizes for putting Sir Henry in so much danger—he didn't expect either the fog or the dog.