Hard to see on screen but may very well be what is known as dictyota which is an algae that has forked branches which may have an iridescent blue hue. Some species of Dictyota are desirable, you will be able to recognize them as they grow as one plant that branches out from one distinct holdfast. Removal should be very simple. Nuisance species of Dictyota, (pretty much all the iridescent species.) stay shorter and creep along the rock. Their branches form straight from the rock, and there is no trunk like feature to the algae, or easily discernible holdfast. For removal, only larger established patches are difficult to remove, treat incoming live rock with it in the dark, or in a separate tank before adding it to the display. If it does make it in your display don't allow it to spread, it is easy to control if it stays managed early on. Take a dental pick and scrape off every inch of holdfast you can.
Cleaners such as emerald Crabs, Turbo snails, Chitons, Limpets, certain tangs, and Urchins will eat it.
