I think it's safe to say that it represents different things to different characters, and that Alan Moore wants you to judge for yourself the meaning behind Rorschach's face.
But I certainly have my opinion. Because the movie is "based on the comic" we can look to the comic for clues. IMHO, this is a question that is answered in the comic book, but not addressed in the movie.
The short answer is that the mask represents "the abyss" (that created Rorschach) staring back at humanity. Chapter VI The Abyss Gazes Also deals with the origin of Rorschach.
Alan Moore continuously describes Rorschach as being above the filth he is looking down at. From Rorschach's journal:
- Beneath me, this awful city...
- The accumulated filth of all their sex and murder will foam up...and
all the whores and politicians will look up and shout 'save us!' and
I'll look down and whisper 'no.'
He's looking down at the abyss, and the abyss is looking back. I would make the case that the abyss is what turns Kovacs into Rorschach, and so the mask represents humanity's evils coming back to judge itself.
In this chapter we see Kovacs being given a Rorschach test, and see the inner working of his thought process.
As the chapter progresses, we see there is an ever changing symmetry in the traumas as Walter Kovacs remembers them. This is the genesis for the name Rorschach.
As Kovacs becomes Rorschach, the case can be made that the mask is the representation of the horrors that have turned him into the self loathing vigilante he's become. That is to say: the mask is the re-projection of the Abyss (that Rorschach is looking into) Rorschach shows to those that he encounters.
To Kovacs the mask is Rorschach's face.
To Rorschach, it's the (symmetrical) horrors of the world that are coming back to judge evil doers. What he calls his freedom "to scrawl own design on this morally blank world." This is the reason the patterns are ever changing. The design changes with "heat & pressure". Heat & pressure being metaphor for the oppressive elements of day to day life.
But to his doctor it represents something different. The horror of meaningless blackness.