From my search it appears to come closest to spiny oak slug, Euclea delphinii. They aren't too picky about food, having a large variety of shrubs and trees they are willing to chow down on. Several online sources list: a variety of deciduous trees and shrubs, not limited to apple, ash, basswood, beech, birch, blueberry, cherry, chestnut, hackberry, hickory, maple, oak, poplar, sycamore, and willow.

Thanks.... I looked for about an hour and finally did figure that out. When I went out to get a better photo, it was gone.
K

I see several types of colorful slugs from time to time. Here's a picture I got of a spiny oak slug - they're quite variable -

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Sherry

I like that one and haven't seen one before!