Dark brown spots on rock with clear fuzz?

I have had some dark brown spots on my rock for months. It is hard to scrape off so I assumed it was coralline. Today I noticed some fuzz growing on those spots. Any idea what it is?

 

Could be a type of algae.

 

Early stage of hair algea.

 

How do I stop it

 

Hermit crabs and emerald crabs will eat it. You will want to lower the nutrient load in your tank by feeding less and cleaning more thoroughly and often. Use GFO in a reactor and do water changes and top offs with rodi water.

 

I already do weekly water changes with RODI and run GFO.

 

Then you want to do the other things listed such as cleaning more thoroughly and more often.

Tell us more about your system, sump, refugium and equipment. Please list your parameters.

Purchasing hermit crabs will be a big help.