Magic Trackpad stuck battery
So the gel containing battery acid has leaked out and dried up.

You need to make it muddy again to be able to remove it, since dry removal is not possible.

There are multiple methods to do that described here. (Vinegar, Lemon juice, and others).

Example:

Mix water and baking soda into a thick paste. Start with about a teaspoon of baking soda and add water a few drops at a time until it can be mixed into a thick paste.

Apply the baking soda paste generously to any corroded areas using cotton swabs. Allow the paste to sit for 15 minutes.

Use a slightly moistened toothbrush to gently agitate the paste. Rinse the toothbrush often, and continue until the majority of the paste has been scrubbed away. In hard to reach spaces use q-tip to deliver the paste.

Needless to say, use gloves!