Looks like Aptasia Anemones to me (although it could be Mojano Anemones). Either way, both are considered pests in your tank (spread quickly, and can kill small animals). To get rid of it, most people get a hypodermic needle filled with kalkwasser, lemon juice, or products called Joe's Juice or Aptasia X. They inject it directly into the stalk. Use the search function and type "Aptasia". Once yours are gone, get a couple peppermint shrimp to keep them at bay.