What happens if a hollow eats a mod soul in candy form
We don't know.

During the run of Bleach, in both manga and anime form, the matter of a mod soul being devoured by a Hollow has (surprisingly) never come up in practical discussion.

What we do know of mod souls is that they were manufactured to help combat the Hollow since they outnumbered the Soul Reapers, but that's...about it.  It also doesn't help matters that the operation which brought them into being was shut down, and all of the mod souls were ordered to be destroyed.