Rhodymenia holmesii

[BlueAbyss]

Hmm, it's hard to tell but I'd almost +1 Dichtyota spp. if it weren't for the color... Dichtyota is a brown algae (though some species look iridescent blue). Other than that, my best guess is some type of Halymenia in an early stage of growth.

These macros aren't generally harmful (might be a little chemical warfare between the Halymenia and stony corals) and some types of Halymenia (the ones that used to be called Gracillaria) are known as 'tang heaven' since tangs love the stuff. My biggest worry with macros is them overgrowing and crowding out corals (I have to prune my Dichtyota regularly or it will overtake some of my zoas).