Brown algae/diatoms? 4.5 year established reef

[flow4show54]

I've got a small bloom of brown algae or diatoms in one corner of my reef tank. Not really sure what to make of it... seems really dense. Mixed 400 gallon reef, running for approximately 4.5 years. Regular weekly water changes. I've seen the normal diatom blooms in new setups but this one has me intrigued and slightly concerned. Any constructive input would be appreciated!

 

[dannysreeftank]

I got something similar a while back when I added some new rock in. Mine took over the rock work but with time it went away. Not a clue if I was lucky and water changes slowly got it or if something ate it

 

[flow4show54]

As of today it seems to have receded back a bit, perhaps just a small bloom after all. Keeping an eye on it though..