Tube anemones do need to have thier tube burried in the sandbed in order for them to be comfortable and happy. They dont have to be vertical, but do bury the tube. They are filter feeders yes, but like most anemones and certain corals, should be fed small frozen meaty foods as well. The green stuff that floats out of them is waste yes.
No one can say if a clownfish will host it or not, as with any anemone, its hit and miss. A clown will host anything if it wants to, even a rock or PH.
Hope this helps
