At night i saw a large flat brown thing about 2.5 inches long and about 1.5 inches wide. It moved along the sand like a nudibranch but as this as paper.
That is a polyclad flatworm, and it will eat any clams and all your snails. You need to try and trap it at night. FW exit will do nothing to it. Good luck...
IME Fw exit didn't work on them, I have only seen it useful against planaria, but others seem to have had success, so it's worth a shot. Keep us posted if the exit works, I would certainly like to know. Good luck.
After reading up it may be a spotted leopard polyclad flatworm. They appear to eat snails and clams. Oddly haven't seem many shells but haven't really looked. I also have a 3-4" maxima still untouched. Been lucky i guess. Most in hobby and more professional articles claim no flatworm RX works on them. They seem to he geared to the types that eat corals.
A trap using snails and a turkey baster and visual sighting may be my limit. Flashlights will send them into hiding. You need red light in order to not spook them, not sure if blue light is the same.
I had a Polyclad flatworm in my 55 and was not able to catch him. I ended up dosing the whole system with prazipro. Fish were fine corals looked annoyed for 3 days. But it got rid of the Polyclad. All sps, fish, lps were fine after about a week.