Yes, "calligraphy" comes from "καλό" and "γραφή".
I haven't seen someone answer your question so I might as well try to explain. I'm not Greek but living in Greece for about 2 years. καλα is simply good. Like if I asked, how are you? you'd say καλα. But καλο you use, if for example you want to say have a good evening (kalo bradi), is the food good (kalo fagito) ? Kalo we use for neutral. And then there's also καλή, we use for fem. for example good woman (καλή γυναίκα) AND also καλός, we use it for masc. good man (καλός άνθρωπος).
Its not a tilde, but an acute accent. Tilde is another diacritic «~» e.g. in the Spanish word «señor». Acute accent is also used in English (French, Spanish and a lot of other languages), like in the French loan words «touché» and «café».
The acute accent marks the stressed syllable of a word.