If it's in the sand and only in the sand, you can remove it by grabbing. I have had it happen that they didn't attach to anything but sand, but rarely. But if it's on the glass you will no doubt leave behind 'spores' (or whatever they are called) that will turn into several more. That's in an easy spot and is big enough to inject with straight vinegar, that's what I do. I happen to have needles from pet vaccinations....but if you don't have needles try aiptasia X or Joe's Juice. People say boiling water sprayed directly on it works too but I have not tried it.
And when your tank finishes cycling (if it isn't), just add a peppermint shrimp. He should eat them and keep your tank aiptasia free. Some people say they have had peps that wouldn't eat them, I never have. They are cheap and cool and great aiptasia eaters. There are other creatures, berghia nudibranch (you don't have enough aiptasia as they eat nothing else), Matted File Fish (they can eat coral if you plan to have coral and are not as reliable as a peppermint shrimp), Copperbanded Butterfly (not hardy and too big for your tank), and Klein's Butterfly (too big for your tank).
Your best bet is biological control for the long haul, and a peppermint shrimp is what you should try first.