Annoyingly it got away and fell through the weir (but didn't fall apart or anything at least). Best case it gets stuck down in the sump and starves I suppose, worst case it gets minced up in the return pump and comes back in pieces which spread. I've looked both nights at all my softies when they've receded at lights out and I can't see any sign of anything on them at all. Reading about them I have had a few turbo snail shells turn up over the last couple of months but I'd assumed it was just old age or something, so I wonder if this particular type of polyclad I have is more interested in the snails, especially given it was up on the back of the tank rather than down by any of the euphyllia. I'll stay vigilant, but I think I'm also going to add in a Springeri Damsel as they apparently eat flatworms, and I've wanted a blue fish for a while anyway. I can't put anything like a six line wrasse in as I already have a fairy wrasse. No idea if the fairy eats them or not. I read mandarins can and I have a slightly fat mandarin so maybe that's why he's fat if he's been picking off what he can.
I'll dip the softies too anyway and see if anything comes off, though unsure what to dip, everyone seems to have different opinions on what dips do and don't work - I have Reef Primer, Coral RX, DipX, Cili Dip in the house already so hopefully one of these is suitable.