Yes, that's grape caulerpa most likely.There are many types..some on runners like racemosa{sp}, feather..etc. Caulerpas have toxins that play havoc with corals. They can also do what is known as "going sexual"..where they drop all the nutrients they have stored back into your tank all at once which can cause it to crash. The tank will cloud up milky white! Chaetomorpha is the easiest to grow, grows quick, good for exporting nutrients when you remove some of it, and does not have the same problems associated with caulerpas.