Halliday & Matthiessen (2014: 195):
The other circumstance in which a clause does not display a Mood + Residue structure is if it is realising a minor speech function. Minor speech functions are exclamations, calls, greetings and alarms.
These speech functions may be realised by a major clause; for example, exclamations by a particular kind of declarative (the exclamative), greetings by an interrogative or imperative. But there are other forms used in these speech functions which are not constructed as propositions or proposals. Many of these do not need to be assigned any internal structure of their own.