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Hi, welcome to the site!
...Red macroalgae. It's too small of a sample for me to really tell which one...it could be gracilaria or botryocladia. I have it all over my snails, it won't hurt them but if you have corals you won't want that red algae...it spreads everywhere.
We don't have coral...
I just recently started a nano 10 gallon. It's cycled and actually doing really well. We noticed about a week after we bought some live rock that this was going... Any ideas what exactly it is?