That's right and you could use either depending on whether or not you have been cleared to descend.
If you were cleared to descend, you descend, but are waiting for someone else to finish their long-winded radio call, then you will have "left' your FL. If you are in fact leaving your FL, you would say you are "leaving." Pretty simple really.
XXX
Pretty simple really ---- simply rubbish ---- with all due respect.
Can we please forget primary school grammar and stick with the very common sense ICAO and the rest of the world recommended usage:
LEFT ( or RIGHT) is a direction.
LEAVING --- when you are proceeding ( nearly said headed) up or down, the associated words being PASSING and MAINTAINING.
Pity there is nothing in the CASA training training syllabi or the AIP about the principles of good R/T, as apposed to a ridiculously (compared to Annex X, Vol 2) long list of "standard phrases".
Tootle pip!!