[Delphinus]

How long has it been the tank?

IME an inverted mouth is usually a response to O2 starvation. Since they can only respire through their skin, this increases the surface area for O2/CO2 exchange. It's generally a sign of stress but sometimes happens in newly acquired/imported/transported/etc. anemones.

Definitely do not attempt to feed it if the mouth is inverting. Even if it takes food (although it likely wouldn't), digesting food isn't its top priority at the moment. If undigested food starts to decompose inside the body cavity it's generally game over for the anemone.

You can try to increase O2 saturation with more water movement, but if he's new the to the tank he might not be firmly attached to anything yet and might get blown around. An air bubbler/diffuser placed nearby might be a suitable alternative..

If he's new and this is a one time thing then he'll probably be OK. If this is happening on and off over days that's generally a poor indicator. I've not really heard that LTA's were prone to this though .. usually carpets right after purchase.

Give the air bubbler a try for a couple hours and see how he responds. Good luck, please post back your updates.