To add a little more info - which I should have done in my first post - this dude was swimming around, not on the glass. If a strombus - do they have a free-swimming phase ? In the final picture I posted you can see 2 protrusions which were used for propulsion - they moved very quickly, almost like a propeller !
Thanks for the suggestions so far !
60G Freshwater - not planted
Rena Filstar XP2 Canister Filter
2" Gravel substrate
10 Danios (mixed Zebra and Gold)