I found plenty of things that will decimate valonia ... they just don't fit in a small tank.
I shutdown my 20g zoanthid tank over it being a losing battle to valonia. Emeralds did nothing - I think once it gets to a certain point those little crabs can't touch the rate of spread of those things in the same way a ravenous tang can.
I have a similar problem with my 40g at the moment but instead of valonia only it's a mixture of valonia and caulerpa.
I rotate some of the rocks into my tank with tangs but for the ones I can't move easily I prune as much as I can with tweezers but it's like draining the ocean with an eyedropper.
I tried a few different urchins in the hopes that maybe that was an idea but so far no luck with that. However I haven't given up on the idea altogether just yet. In my other tank I have 2 green urchins that seem to go after valonia, I am hoping to find more of these but I don't know the species ID. If you google green urchin you can find some coldwater species that look sort of correct but I've had one of them for 5 years now easily so if it is a coldwater species, it seems to be one who can adapt to warmer temps but I suspect it's probably a different species.
Anyhow I nod very sympathetically in your direction Myka. Good luck!