The little bit we can see at of the underside isn't quite enough, at least for me, maybe there's some out there with a keener eye than I have
If I had to guess (and I'm not very good at it, so don't take my word - hopefully someone else will chime in who is good at it!) I would say crispa, here's why.
Placement - it's between rocks, but looks like it's probably hitting sand too. Crispas are mainly sand dwelling nems. I don't think that magnifica's are, I think they like to be at the top of the rocks.
Tentacles - crispas usually have quite a few, I think more than a bta - but again, I could be wrong. I've only read about bta's and seen a few in person and then pics. On the other hand, looking at the first picture the tentacles throw me off a bit with their placement. My crispa has tentacles that come almost completely to the mouth, yours seems to have a definate ring around the mouth that is tentacle free. I don't know enough to know if this is just a variation in crispas or if it's a distinctive feature between nem types.
Underside - the little bit of disk that I can see there appears to be some verrucae (I never know if I'm spelling that right, but it's the adhesive bumps you would see on the underside of some anemones). Unfortunately, more than just crispa have these bumps, and the pattern is what helps in correctly identifying the anemone. BTA's if I remember correctly, don't have this.
Do you remember:
the color of the foot/column
if the verrucae (assuming that's what I see) are sticky?
Also, are the tips of the nem colored? I can't tell from the pic. The most common color for the tips of a crispa are purple, or variations of it. Sometimes though, they'll have green tips - I know of two people on the board have green tipped crispas.
It may be possible that the nem is a Long Tentacle Anemone (lta) it's hard for me to tell the difference sometimes, but usually their foot is orange/red and I think the verrucae have a different pattern than the crispas, which is why a shot of the underside would be helpful.
Hopefully this was a little bit helpful and only had minor errors, not major ones. I'm still learning how to identify nems, so like I said, I hope someone with better experience will jump in.
Good luck!