It is its short/weak form. https://en.wikipedia.org/wiki/Modern_Greek_grammar#Personal_pronouns
Because it is the object of the verb it is indeed in the accusative.
The old Mandeson (1961) dictionary lists this verb under λέγω and indicates that it has two forms λέγω και λέω. People who come to modern Gk after having studied ancient Gk are used to the gamma being there. Does anyone know if the gamma is heard or spoken with this verb more often in some parts of Greece than others? Is it a dialectical difference to use the gamma or not?
Personally, I’ve rarely heard “λέγω” in everyday speech. It’s quite an oldfashioned form. The only contexts in which I can imagine it being used is in very old texts, biblical or other religious texts, or maybe used by some old people in speech. It would definitely sound odd to a modern Greek person’s ear in most cases though :)