No offense, but I'm trying to understand this. I can't hardly tell what you're trying to say. I'll try to translate...
my water parameter are good
Example? Many people who think they are good are quite a bit off. What are you using to test? What brand?
the light just been use for 2 months
2 month old light, should be good. What bulbs?
my eating silverside and squid and oyster once every 2 day
Try Mysis and soaking your food in Selcon, VitaChem, and other boosters as well. The color is not one of a healthy anemone (should be dead quite a while ago actually) and appears to not be eating. Do you see it EAT the food and "poop" out digested material? Does it expel it later undigested?
my had been in my tank for 6 months now, the color when I got it is much better
No clue what this means. Can't tell if it's better or worse. It can't really be worse, because it has NO color now, so I assume it's "much worse". This is a sign something is terribly wrong with your system or setup.
The problem is my rbta alway hiding under the rock, when I flip the rock to make it expose to light, next day it go hiding again
light is not a problem the problem is it alway hiding from the light, how do I fix that???
That in itself shows a major issue. Your anemone is going to move where it feels comfortable, and you are stressing it greatly moving it. It will come out when it's ready.
36" Ready Fit: 2-39W T5 HO Retrofit Kit
tank 55g tall, I don't think my light is too intense at all
I'm not familiar with this size, but I think this is one of your biggest issues. What are the dimensions of your tank? BTA's have pretty decent lighting requirement, and RBTA's seem a bit higher. I would have a minimum of 4x 39w with individual reflectors (non HO). You're probably cooking it with two super bright lights that are not creating a good spread throughout the 55 like 4x 39 non HO would.