My theory is that the Slinker is actually Spinel.

The Slinker has impressive vitality and strength. It hid very well. It also poofed Amethyst many times as if targeting her.

The gem Spinel. Promotes Vitality and Strength. It's also called the imposter stone. This would be good for hiding. It promotes a fresh start and new beginning. It made a new beginning for Amethyst. Spinel is a stone of confidence. Every time it poofed Amethyst it was doing it to her for a reason. It was trying to make her be confident in who she was. That's why it was in her room and targeted her. It was giving her the change she needed.
That's also why after she poofed the last time, it left. She still goes in her room but the Slinker doesn't attack her. It doesn't have a reason to. She's finally becoming how she actually wants to be.

Side Theory: I don't t think that the Corrupted Monsters are actually monsters. They are just acting on their gems properties. They don't mean any harm. They actually mean to help the gems. Wether it be as a whole or an individual, they want to help.