What are your water parameters? (ammonia, nitrate, nitrite, salt, temp etc)
Is this a newly acquired coral? If so, it's normal for a leather coral to shrink up and appear slightly green.
However, if the leather coral begins to "melt" I would dispose of it immediately. A dying/melting leather coral in a 30g display may be a disaster. I've found them to be rather toxic in small water volumes.
You may want to run carbon.