We're so lazy that we can't even be bothered to type "character" half the time, so "grapheme cluster" has no chance to catch on unless we can think of a "char"-style abbreviation. I'll start the bikeshed with "gracl", but ambitious folks may wish to argue for "clog" ("cluster of graphemes").

Unicode uses the term grapheme cluster to emphasize the fact that it can be composed of a cluster of codepoints, and perhaps also to distinguish it from the linguistic term grapheme. But ultimately in the context of Unicode I think you could just get away with using grapheme. That's what Perl did when it invented Normalization Form Grapheme (NFG).