Is the phrase "re-heated leftover Mahi-Mahi" considered a redundant pleonasm or a plethoric circumlocution?

I wouldn't say that it was either. Redundant pleonasms are phrases or clauses which use redundant words. A commonly used example is, "I saw it with my own two eyes": the act of seeing is done with the eyes, so there is no need to specify this. Plethoric circumlocutions are instances where more words than necessary (a plethora of words, hence the name) are used to describe something. So, to identify if either of these is present in the phrase, "re-heated leftover Mahi-Mahi", we're looking to see if any of the information is redundant, or unnecessary. I think that this phrase contains neither. "Re-heated" specifies that the Mahi-Mahi is hot, not cold or room temperature and "leftover" specifies that the Mahi-Mahi isn't freshly made. So all the words used are relevant. This tells us that it isn't a redundant pleonasm. There are a suitable number of words involved, so it can't be a plethoric circumlocution, either. Hope this helped!