Mexican turbo snails and tuxedo urchins
Algae in a tank is normal. It gets out of control when it is not properly managed with clean up crew and nutrient levels
That will kill any beneficial bacteria . If it hasn’t been in the RO water long , return it to salt water . Get your parameters in check and follow earlier instructions. I wouldn’t go the chemical route and nothing good happens quick in this hobby . Patients is your friend.
Assuming you were to kill the algae with peroxide ,algaecide , or any other measure and since you’ve put the rock in RO water you will have to cycle it again and if your parameters are not in check your issue with GHA can come back . You may in-fact have been doing this a long time and certainly I’m not trying to belittle your experience but you chimed in for help and that’s what my goal is . I wish you the best of luck !
If you already have the rock out of the tank best thing to do is use some good old elbow grease and a brush
This and try a total blackout for 3-4 days .the peroxide and a good brushing should do the trick . I won’t advocate the use of a algaecide
Plus shorten your light time and or intensity, keep your nutrients low but never let them hit zero , and manual trimming/ pulling
