That was her Kymaera form. It's a result of genetic meddling; when she was artificially created by atlantean scientists, they used dormant genes of ancient atlantean warriors to make her a specimen of their kind. As a result, she would turn blue due to her emotional/psychological state. The current Namorita is from the past, as explained by Digifiend, so she hasn't gone Kymaera yet. But if she is the same character, it could happen any time now. I hope they bring this concept back, as I really liked it.
I've always assumed that she is referred to as Namor's cousin because he is her first cousin once removed (someone who is the first cousin of your parent), but saying ''first cousin once removed'' is kind of a mouthful, so it's easier to just say cousin. I do think Namora is considered her mother, because regardless of the cloning aspect, it's the kind of relationship they shared while they were together. Much like Laura Kinney is technically also a clone of Logan, but she has been treated like his daughter over the years.
Yeah, I hope they actually try to make sense of everything that happened to Nita over the years, but with this allegedly being just a mini, I'm afraid there won't be time for that.