You're right, that's a lot of depth for a
plate coral.
Your clown hasn't shown any interest in the frogspawn, eh? Well, there's no telling what they'll take up with as a subsitute host for their anemone. Since you already have a frogspawn, maybe a bubble would be a good choice. You can also always go for something simpler like frilly/fuzzy mushrooms, Ricordea polyps or star polyp if you don't have any already. I've had clowns host in all of those. As well as an open brain, sun polyps,
feather duster...like I said, no telling!
~Alice