Why is your light so low?
What do you mean it has grown a lot? It is only one polyp. In 3 months time it should have several new polyps. Its not growing at all, and I believe its dying of light deprivation.
Turn your lights up! Find out what other people have their Kessil A160 set on on a similar sized tank and copy them.
Nitrate and phosphate is also too low.
Low light and low nutrients.
Do you have any flow? It likes flow, it likes to wave back and forth.