Hi! I’ve had these cacti since May, never been repotted and have been absolutely fine for months - I believe they’re ‘toothpick’ cactus but i’m not 100% sure In the past couple of weeks this one has mysteriously started to shrink and turn black (for reference it was the same size as the others) Does anyone know why? Or if it can be saved? The other two are absolutely fine and no sign of doing the same at all! Thanks in advance :)