Photo by James Florence
The silvertip nudibranch feeds on bryozoans, like the spiral moss animal Menipea triserata, a vaguely plant-look-alike colony of minute animals. It is somewhat unlikely that this young silvertip crawled onto the anemone, and probably was wafted there by a surge. I have often seen nudibranchs floating around in mid-water, and believe that not all of them are there by accident - instead, I think it is a very efficient way of moving large distances in search of food. It may be a gamble, as it could lend up on a barren sand-field, but it could equally land it in a richer environment than it was on previously. As nudibranchs are slow crawlers, they can only cover so much distance, so this tactic is quite useful. Due to their unpleasant taste fish invariably leave them alone, and while they often show an interest, they must have learned not to bother with these brightly-coloured little blobs.
From the photo it does not seem that the anemone is interested in the nudibranch either - if it had been it would have contracted, to make sure its meal would not simply "swim" away. Being fully open it probably doesn't even know it is there.
Two Oceans, A guide to the marine life of Southern Africa, GM Branch, CL Griffiths, ML Branch and LE Beckley, 1994.