It’s currently not possible at all to report dynamic heads via the Roblox mobile app on iOS. You can report any other accessory type (from what I’ve tried) except dynamic heads (from what I’ve noticed).
Expected behavior
We should be able to report dynamic heads such as the one below which is obviously not appropriate for Roblox.
As you can see, there’s no option on the quick menu or full item page to report the Dynamic Head via Mobile. I’m using the latest version of iOS and the Roblox application.