Looks like some red grape caulerpa. I occasionally notice some in my tank once in a while. Usually if your phosphates and nitrates are kept in check it won't become too much of a problem but if you can I would try to get rid of it. I had the green grape stuff before and it got way too out of hand. You would try to pull it out, but ripping it would leave its roots behind so it would always come back. It finally got to the point that I got rid of any rock with it.
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Rory
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