I mean Lily's ghost (don't think this is a spoiler) - almost looked like she was chewing on pieces of string. Confusing random detail in an otherwise enjoyable, easy-to-understand film.
The image very much mirrors the scene in which she is chewing on the telephone cable right after the phone was snapped out of her hands (same gesture, same framing, same clothes), which marked the first time that the ghost of Polly clearly tried to reach out to her. You may draw your own conclusions from that.
You might say that her spirit is to an extent perpetually stuck in that moment, or if you want to read it more symbolically it's an image for how she is perpetually scared of the ghost of Polly, being scared of Polly's ghost was after all what killed her. Personally I might even go further, since the film makes allusions to there being a connection between Polly and Lily I'd say that they are semi-united in the spirit world, so she is stuck in the house and reliving those experiences, basically scared of herself.
"Let's went, before we are dancing at the end of a rope, without music."
My thought (I watched it again and it seemed right) was that it was her sweater, when she's staring at the ghost her mouth is doing something weird like she's gasping for breath, my thought was that she was kind of suffocating herself, and when she looks back to see how she died she gets a glimpse of what really happened, since at the time she was unaware that she was doing it.
We are looking at two images at different angles superimposed upon each other and one appears to be in slow motion. Both images are reliving her death. When she was dying she was opening and closing her mouth and pulling on her sweater. The sweater she is pulling on in one image (the slo-mo image) appears to be going into her chomping mouth, which is actually the other image. I had to watch her death scene and the ghost death reinactment a few times to figure it out.