They are two different genera belonging to the family Mussidae. Micromussa has smaller polyps and I find is generally less aggressive in its' feeding habits than is Acanthastrea.
one other question where should my acans be located at in my tank. right now they are on the bottom of my 90gl and under (2) 250w 14K MH. does this sound right? and how much would a colony this size of micros go for?
Nice scores alright. I remember reading in a discussion about the main difference of acans vs micromussas and as stated it is the size of the adult polyp, micros being 7mm and below and acans being 10 mm and above.