That anemone looks like it is a BTA in very poor shape. In anemone world, BTA is practically almost indestructible in term of ease of care in comparison to other host anemone species. It does not appear to have adequate light or circulation there, but likely that is not make it sick. Anemone just moves if they don't like the current and light. I almost 100% sure that that the problem is in the chemistry.
the tank is about 2.5 years old now, i haven’t done any changes to the tank like water changes for about 2 months just been topping the tank up with water and added in chemicals like ph
The OP mention that the tank has been running for 2.5 years. There was no water change in the last 2.5 months. Poor maintained tank has "old tank syndrome" which mean basically "bad water due to accumulation of micro-toxin, and depletion of micronutrients. Turn this around isn't always easy because a lot of the micro-toxin bind into the rock, only release when pH gets low like during the night. Because of this reason, large water changes do not always fix things. This, "old tank syndrome", is a lot more common in tank with sand bed that have been neglected.
....when the zoas started to shrink and anemone became bleached i did multiple water test at the shop on their machine and everything has always been perfect just upping my salt sometimes and more RO water depending what the test said. have been feeding the anemone at least 3 times a week and always ate the shrimp i had given it
LFS alway just test for ammonia, Nitrate, Nitrite, salinity, perhaps phosphate. Testing at the LFS is hardly extensive. It will not pick up cause for "old tank syndrome".
OP mention "...upping my salt and more RO water..." I just hope this does not mean adding salt crystal to the tank.
Concentrate on get the water condition better, however you can do it, or else get him to a new well-run tank and you may be able to save him. When I am talking about getting water condition better, I do not mean just salinity, nitrates, phosphate, but more than the macro nutrient and waste, I mainly concern with the normally unmeasurable trace nutrients and toxins. You
MAY be able to correct this with large volume water change, 50-75% changes repeatedly, or else 100% change but you must match salinity and temperature.
Goodluck