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If you put SW LR into a FW tank you will kill everything on the rock. The bacteria is similar to what develops in a FW tank but it is SW rather then FW. I have no idea what kind of reaction your fish would have to the introduction of SW to their tank. I have used aquarium salt in my FW tanks but I'm pretty sure that is different then ocean SW.
If you are wanting surface area for bacteria to colonize then get some dry base rock for your tank and let it develop in the FW.
I don't think it is the rock that promotes algae growth. Algae will grow based on your water parameters. I'm not up on FW keeping so you'll have to get another member's opinion on that issue.