Question
Today I notices a white ball growing on my a liverock in my aquarium. It only appears on the piece that was previously in my sump, should I be concerned?
Answer
Hard to tell from the picture, but most likely some sort of sponge or tunicate. They typically grow naturally when there is sufficient food and nutrients and with a few exceptions shouldn't be a problem.
Here's a quick guide to common hitchhikers: http://www.xtalworld.com/Aquarium/hitchfaq.htm
Sponges come in just about any color and shape, the most common are ball and pineapple sponges.
Answered By - Jestep