Nubis look for them under cap if possible. At night when the lights are off look in tank with a flash light you will see them eating. Six line wrasse can help eat them. Important to dip Monti to kill them but the eggs it won't kill. So you got to do it again when the eggs hatch. You might be better off QT the Monti and keep a closer eye on it. If you don't see them maybe something stinging the monti. Make sure you cut it off.
most likely Montipora Eating Nudibranchs aka MEN. They are difficult to get rid of. Hopefully you can eradicate them. I lost all of my montis when i had them. I kept the tank free of montis for months and they died out. Best of luck. I would try bayer.
Bayer dips once every 5 days for a month at minimum to beat their life cycle. I had them one one new frag and now will be quarantining every frag for at least 2 weeks minimum as a week wasn't sufficient for inspection last time. It's hard to wait, but worth it.