Sally I agree with Rishiwizzz, I don't think it was "ghost mum" it was part of Pearl's own coping mechanism and a way for her to process the whole situation.
qalilah I don't think that it was her mother in a ghost form. I think it was that part of her that longed for her mum that came to life. About that last bit, Pearl is now accepting that her mum is gone and won't come back, this time, for real. She's repeating that "gone" bit again and again because she's finalizing that to herself.