You've had your sebae for quite a while now, and he's getting smaller.
That's a sign that it's ill, or not getting the nutrients it needs on it's own. Sadly, he may die, but maybe it's not to late! I have a sebae anemone, and he came to us about the size of a baseball. He's now as big around as a volleyball. The sebae was white with purple tips...which most LFS stores will assure that it means they are healthy. A white sebae is a dead sebae in the waiting. Their natural & healthy color is actually a pinkish brown color. That color was actually achieved after hand feeding him weekly with brine shrimp. Mine wouldn't eat either at first probably since no one ever fed one out in nature. Take something out of nature, and you should expect to adapt a way to feed it if it doesn't thrive on it's own. I'm a sworn hand feeder to a few special inhabitants of our tank. They are all doing excellent since I figured out what their needs were to survive out of the ocean. Other people have great luck letting their anemones feed off what's in the water column. Not me, so I "baby" my critters as I'm told.
I taught him to eat by feeding him "frozen brine shrimp". I put a chunk in a bowl, added tank water to melt it, and make sure it's room temperature. At first I would take a bit of the shrimp water and spray that on him. He learned it was time to eat this way. To this day, I feed him once a week. He's up to a 1/2 full baster of brine shrimp, and he won't waste any! As soon as the baster touches his tentacles, he's senses it's food time. I can see his mouth now, so I poke the baster gently down into his fleshy middle into his flesh and slowly plunge in the food. He brings his sides up around the baster and food, and doesn't let any of the shrimp escape.
I have tried silversides with him, and he eats them now....but only if they are chopped up into small bits. When he was small, he didn't eat them at all. They would just become waste in the tank in a hurry if your clean up crew isn't on the ball. He handles 1/4-1/2 inch bits of silversides, but the all time favorite is his brine shrimp. I imagine if I had blended them, he may have eaten silversides as well. I have heard other reefers say that they feed their anemones a mush with a blend of many different foods to insure he gets all of his health needs. It's good advice. Since mine is so big now, I feel assured that he's thriving and healthy. He doesn't seem to have any interest in eating my tank's fish or anything other than his food, he's a healthy pinkish brown with blue-green tips, and that's a good thing!
Maybe trying some of my ideas will work with your sebae. I'd leave him in the same tank he's been in, and maybe block off the area you're feeding in so he doesn't get bothered by always hungry fish trying to steal his food. You may want to sprinkle some flake for the fish to keep them busy. Remember, anemones tentacles are sticky, and it will hold onto it's food if it's healthy. Keep him out of rapid flow so the food doesn't blow away while you are teaching him to eat...mine likes the corner.... and they do like bright light! If he doesn't like where he's placed, don't worry....he'll move somewhere that he's happy. Your patience, persistence & care are just what your sebae needs. Good Luck! .....Angel