It could be a few different things or a combination causing them to turn green.
Some things to look into (I'd start with the first one and work your way down)
1. PAR is insufficient. Usually colorful acros need very high PAR (and flow) for to thrive and develop and or keep these beautiful colorations.
2. Nutrients issue (usually too high). When nutrients are in abundance, corals commonly lose their bright and colorful colors and just turn green or brown.
3. Your light spectrum might have too much white/greens. In most cases, very heavy blues and very little to no white/yellow, red, and green spectrum will help bring out better colors in corals.