It's not rare... just seldom imported. You have to shop around a bit to see if the prices are suitable for you. Some stuff come from further away... so there is a price difference.
The 'rare white stuff' is just a joke amongst us experienced reefers who sometimes see LFS people selling to newbies some bleached corals... well... everyone is a newbie before... experience and knowledge is therefore important to avoid selecting not so prime specimens!
Oh, btw, xenias don't bleach... they just melt... They come in all sorts of colours and sometimes, people may ID them wrongly or mistake them for clove corals. Oh... and not all xenias pulse...