If "hexa-" is a prefix representing six

Hexa- is the Greek prefix for the number six, from hex, "six"; cf. hexagon, hexameter, hexad, etc.

Decimus is the Latin ordinal number "tenth"; cf. decimate, decimal.

This hybrid construction hexadecimal is strange but often seen in English to mean "sixteenth" or "pertaining to sixteen". It does not exist in either Latin or Greek, of course.

In Latin, it would be sedecimus, "sixteenth", leading to English sedecimal.

In Greek, in would be hekkaidekatos, "sixteenth", possibly leading to English heccaedecatic; but derivations of such polysyllabic Greek numbers are rarely used in English. The prefix would be heccaedeca-, as in a heccaedeca(h)edron, a polyhedron with sixteen surfaces.