I have been on the hunt for Ceriths! JoJo is right, a bunch of those are great.Pitho crabs are quite useful an, in my experience, quite safe.
I also like having an army (~50 or so) of dwarf ceriths. They can't do anything about that GHA there, but they get into the very tight slaces and might be able to prevent it taking root elsewhere.
Tux urchin is great and very fun, but if that doesn't get the job done, 2 Mexican turbos will do it. Bulldozers, yes, but very effective.