Сейчас у меня [есть] мало времени
is correct as already pointed out, save for the use of the verb есть, which here is only implied.
In this sentence Genitive case stems from the relationship between the quantitative pronoun мало and the noun время and not from the implied verb есть.
So without the pronoun the case is Nominal as expected and with the verb есть intact
Сейчас у меня есть время
However with the pronoun мало, which is the bare subject of the sentence, the Genitive form of время -> времени answers the question (мало) чего? - (little) of what?
Likewise with other quantitative pronouns много, немного, немало, столько, несколько, сколько?, also чуть-чуть and полно́ which function as pronouns, and with synonyms of those.