Terve means 'healthy' [health = terveys]. Tervetuloa is a compound word of 'terve' & 'tulo'(-a), as in "healthy coming/arriving".
'Moi/-kka' is very common in everyday use, but would be closer to the short and informal 'hi'. For 'hello', I'd stick with 'terve' or 'hei' as they're rather neutral (not formal or too informal). :)
"Terve" is used as a greeting just like "hei" or "moi". Perhaps they decided to have it as the greeting here in the beginning since it actually means something in Finnish and thus "feels more Finnish" than "hei" or "moi" which are of Germanic origin. But yes, "hei", "moi" and "moikka," etc. are all valid translations for "hello".