PD 1 employs "aphtharton", as mentioned above. Perhaps this is Epicurus' preferred word when describing gods?
Vatican saying 78 uses "athanaton", speaking of immortal good.
The Letter to Menoeceus uses "athanatois", a slight variation of the same word. This change reflects the agreement of the word with the plural "agathois" (goods). "Agathon", singular, is used in the previous formulation.
Both relevant words, aphtharton and athanaton, are formed by prefixing the word stem with the negation "a-". Same here as in English; atheist, amoral, abiogenesis.
Phtharton is defined in the "Middle Liddell" (a scholarly lexicon of Ancient Greek) as "corrupted; decaying". Aphtharton, then, is uncorrupted, and undecaying.
Thanaton (-os), as Hiram mentions above, is death. Athanaton is immortal, or deathless. So there are evident shades of meaning between the two.