Brownish or blackish bubble on my live rock

Question
I have a 125 gallon saltwater tank with live rock in it.  Recently I found what appears to be a small balloon shape bubble looking thing attached to my live rock, there is only one and I have no idea what it is???? Nobody does, can you help me please.  It is about 1/4 inch diameter.

Answer
Hi Melissa. It sounds to me like you have what is known as bubble algae. Usually these will appear on live rock and be a greenish color however I have seen them so dark green that they appear almost black. This algae will bloom in a tank under nutrient rich conditions and are not really a cause for concern. Simply pick the bubble gently off the rock and toss. Inside the walls of this algae are thousands of tiny newly formed daughter cells that when popped will form new bubble algae. Don't be too concerned if it does happen to pop however even if new ones form they rarely populate the tank enough to harm anything. Emerald crabs are excellent for controlling this algae and are perfectly reef safe.