Some scale insects, specifically the 'soft' scales, do not have the protective shell associated with scalebugs.

The soft bodies resemble a slug.

Canker is the term for a plant's response to a specific canker-causing bacterial infection. It isn't an insect.

Did you know that slugs are hemaphroditic? Some species can even self- fertilize, while other species display a behavior that involves one slug biting off the penis of the other. The second slug then has no choice but to behave as a female since it can no longer deliver sperm.

Anyway I still have my money on soft scale.