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Amphiscolops sp. flatworm. Harmless, though sometimes they can become a nuisance. Right now, if you are only seeing 1 or a few, I wouldn't worry about them.
Not to rain on your parade or anything... but I bet if you look closer, you'll find more!
(I thought I just had one too. While I'm not at plague proportions, I am doing some mid-week syphoning of them to make sure they don't get away from me. Just keep an eye out...)
There numbers will rise and fall depending on their food source. I used to have them and never had them get out of control. Mine eventually died out on their own. They love to be in and around macros. (I'm pretty certain they consume copepods which would be abundant in clumps of macro algaes, and a quick google or search of wetwebmedia.com could confirm that) I wouldn't be overly concerned about them.