The two curved ones in the circle are tubeworms. The one in the circle far left is suspect and might be a nudibranch. The tiny specs to the right are also suspect and might be eggs. Based on what you described, you most likely have nudibranches. You might be lucky and just have spots from detritus collecting on the plate causing some tissue necrosis in that spot. Flow over that plate would solve that.
Nudibraches are white and tend to be about the same size as what you have circled, and can look almost the same. The main difference is the nudibranch is soft and the tubeworm tube is hard.
I just found nudibranches on my leaf plate monti and am going to attempt the battle, but I hear from others I will lose. I only have one leaf plate and one monti setosa so I don't have a lot of monti's to lose.
Using a pipette I squeezed out the air and used the end to scrape the objects off the underside of the monti while letting the loosened objects get sucked into the pipette. Here is what I found when I looked at each one under a microscope:
- tubeworm tubes
- sponges
- nudibranch
- nudibranch eggs
- detritus
All five objects had a similar appearance because they are so small. Once you extract them you begin to see the difference under magnification.
Try tweezers or a pipette to test each white spot to see if any of them are soft. Try to get a sample of any soft ones and put them onto a black plastic surface with a drop of saltwater and look at them under a magnifying glass. If it's a nudibranch you will see it moving if you watch very close.
Once you confirm you have them, or hopefully verify you don't have them, you can act accordingly.
