Sorry to hear of your mishap. I had a mertens nuke my tank, killing all my fish but clowns too. Quite a few, tangs I had for years. I know how you feel. I really do. Painful to see all your fish in the sand, I know how it feels...
So, was this nem on an island type of rock? I also use MP40's, and using sponges, well, I've used them too, see above... I no longer use sponges, worthless in my experience... Although, a friend just posted he had a nem save because of them, so every individual has different experiences with them. I continue to use mp40's, no screens, but have just gotten a more keen eye as to happy or unhappy nems... (carpets tend to move slower..) And, I keep them at a distance it would be hard to reach in 12 hours left unattended. The imaginary line in the sand has moved as to when I "do something" or I let it ride.
So, how was the rock work yours was on? Island style that everyone says is what mags want? Just curious how it happened. Us owners get comfy in how they are "happy", if they don't move for a long time, but I've seen the same thing. Established nem going for a stroll... Again, sorry to hear.