"green tree coral" is by NO means enough data for any of us to give you more info on. Is it an SPS, LPS, NPS, softie, shroom, etc?? . A picture would be worth 1000 words here - but 40 watts of light over a 55g tank is basiclly nothing - I would hope your green tree coral is an NPS (non-photo species) item - otherwise, you havn't enough light to keep anything coral wise.