Well, it looks like it's a valid GUID string with 32 hex digits, but it is not.
When copying your string into an editor I see that the very last character is unicode point 8207, Rigth to left mark. It's an unprintable character, but it's there. Try this instead:
new Guid("31033981b158e31187e700155d094430");
You can't see the difference in your browser, but it will work - I removed the unprintable character from your string.