MOROMISATO seems to be a popular but very localized family name in Okinawa. Because the name is somehow popular, it would be difficult to find your relatives just only from the family name. I am not a specialist for family lines in Okinawa, but MOROMISATO can belong to ITOSU family (http://www.okinawajoho.net/pc/culture/kamon/pages/a01.html
; you cannot read it probably, sorry). And MOROMISATO is a name of a village of Okinawa. That's all what I searched. Good luck.