White colored, cerith (pointed cone shaped) and about 3 to 5 mm long could easily be pyramidellae snails. You need a positive id on them before you kill a bunch of baby cerith snails. They are very hard to tell apart. If they are the pyramids, they bore into things such as tubastrea (lps stoneys and that can include torches, frogspawn, etc.) and clams, and kill them for food. They love clams!
That caterpillar thing sounds like a bristle worm and the rest other detrivore worms too. They're out cleaning up the place as are the snails.

If you can capture a bunch of them, I can swing by or meet you somewhere tomorrow and id them for you. Get a good sampling and look close at the foot. It could be both kinds or even another kind of snail. There are several snails that look like the pyramids that are not. Don't get bit by them. There is one kind that is cone shaped that injects poison with a probiscus into its unsuspecting victims.

Chitons ok, unless they are nested in a poccilopora colony and then they may eat the growing tips off the coral as they graze across the top. That usually doesn't happen, but it can. Chitons normally live on and in a rock making little channels across it for their paths or burrow into the sand along a rock or on the glass. They eat algae for the most part.

There are worms that eat algae and others that eat sponges, some that eat rocks (those are boring) but most eat detritus and left over food.

Really, the above is true. Are you scared yet? Focus on the snails and let me know about meeting you for an id.