The algae looks like bryopsis as mentioned above. There is a lengthy post in regard to it taking over tanks and treatment options. IMHO I would discard the rock and pickup a new Piece of LR. I have learned the hard way with a chunk of LR with aptasia. You will end up spending a lot of time and money trying to remove bryopsis. Hopefully you find the answer you want in moving forward.
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300g Basement Reef - April 2018
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