When it finishes it will head directly to the next closest doomed plant. After it eats that one it will again leave a slime trail to the closest remaining plant and so on until the garden is no more.
Where are the 4 remaining seedlings if the path crawled by the slug is the longest possible and it never has to choose between two equidistant snacks?
Note: Although the slug will never have to choose between two equidistant seedlings, this doesn't imply that no two are equidistant.
Next find the locations if 6 seedlings had germinated instead of 5.