I would argue that it is rather difficult to identify one dominant mood in a work of literature. The majority of texts have a number of different moods as the story is told, which can range from despair to humour. However, if we think about this text, perhaps we can argue that the dominant mood is one of nostalgic remembrance as Elaine seeks to go back into the past and come to terms with events in her life surrounding her friendship with Cordelia and how it affected her and still affects her now in her adult life. Note the way that the story begins with a focus on time:
But i began then to think of time as having a shape, something you could see, like a series of liquid transparencies, one laid on top of another. You don't look back along time but down through it, like water. Sometimes this comes to the surface, sometimes that, sometimes nothing. Nothing goes away.
This is of course a summary of how the rest of the story develops, as Elaine tells her story from what emerges from the lake of time in a series of flashbacks. Elaine if you like resurrects these memories, dragging them out from the lake of her past so that she can live through them once more and by doing so, somehow make peace with Cordelia after all of these years. Elaine at no point downplays the trauma of her "friendship" with Cordelia, but there is at the same time a desire to try and heal the damage that has been done.