Hello! You have some interesting hitchhikers!
For your video on post # 46 - that is some sort of blue Dorid nudibranch
For post # 48 - the transparent coneshaped things are tunicates. As for your question on nudi food? Some species of nudibranchs eat tunicates, but I'm not sure if yours does, as I haven't ID'd it to the species level.
Tunicates are filter feeders. Good CUC, if you can keep them alive. Many will slowly die off in aquariums.
http://depts.washington.edu/fhlk12/links/StudentProjects/Tun.biology.html
http://www.mesa.edu.au/tunicates/
For post #49, now that is interesting. Tell me is it still there? Has it moved at all? I'm leaning towards it being a mollusk of some type (mollusca). Does it have a little tiny shell on its back?
For your posts questioning the snails on the previous page, can you get better close up photos of those snails?