Hello Goncin,
that's true είσαι is the present tense, 2nd person singular of the verb to be, but "Πως είσαι σήμεÏα;" means "como estas hoje?". In english the phrase "How are you?" means how does a person look, what is his appearance , for example a possible answer would be "i am tall/ i am beautiful/ i am slim ect. If they want to ask how does he feel they ask how is he doing.