I may even go bigger than that tank, and remember your tankmates will be VERY limited. Green morays grow extremely fast and will quickly take out any tankmates, not to mention take out a few fingers if you try to move things in their tank.
The green moray gets as large as 10 feet in the wild, and grows up to 6-8 feet in captivity. So you'll want a very secure top, a canopy you can clamp down with vice-clamps or heavy cinder blocks.
Someone used to post pics of their green, they commented their 125 was too small I believe...paging Polarpooch....