There is not a single place with all the information about anemone species. Most of the information you just going to come up with reading about the species at various source on the internet.
What I do is to read and see where they are in the wild. From where they are found in the wild, I can deduce what kind of environment they need. Read as much as you can about how other people keep their animals. These information will have a lot of contradictions. With reading you will come up with a sense of who are the more reliable and who among us are not as reliable on the information they give.
In short. You just have to know enough to judge on which advices to follow.
Your anemone does not seem to be sick in term of infection. It seem that your tank is too now and it is not stable enough for anemone.
Haddoni need lowish current and bright light. It need stable good water with relatively low in nutrients and a sand bed of about 2 + inches.
I would do water changes but you got to mix the water at least 24 hrs in advances. Make sure the salinity is match and temperature reasonably the same. More likely than not your anemone is not happy with the water chemistry. Test your water, and do water change 25-50 % every few days.
Hopefully this will turn it around and you able to save a very beautiful Green Haddoni. You will not find a better green color anemone than your anemone.