"Do you have a good memory?"
Translation:Έχεις καλή μνήμη;
You do not use the indefinite article if the following noun is 1) abstract 2) uncountable. Memory (in the meaning of mental retention, not in the meaning of specific recollection, which in greek would be ανάμνηση), is both abstract and uncountable.
Part of your confussion might be because the sentence in English is ambiguous. "Do you have a good memory" could mean "are you good at remembering things" OR "is there a single memory you have that is good".
In Greek this ambiguity is not there and the sentence can only mean "are you good at remembering things"