It looks like small sea anemone looking creatures. They multiply like crazy, live rock are the main carriers for them. They are worse than fish ich, Aiptasia can destroy the whole aquarium eco-system. When we first get into the hobby we really like these things as we think they are sea anemone or a feather duster, but in reality they are the pests. They grow in numbers and these numbers together will absorb most of the water nutrients, causing stress in the other live organisms. They even sting others to death.
Aiptasia can reproduce both sexually and asexually. A single cell can be enough for a new Aiptasia anemone to form. They can grow everyware from rock to sand to glass even filter sumps. They are hardy in nature and can tolerate wide variation in water parameter.
Once in your system, the only way to get them out is to redo it. If you don’t have Aiptasia in your system then prevent it by quarantining the live stock you get use dips which are widely available in the market. If you have it in your system then you can try to control the out break by either Aiptasia eating live stock which are Peppermint Shrimp (Lysmata wurdemanni), Copperband Butterflyfish (Chelmon rostratus) or the
Aiptasia Eating Filefish (Acreichthys tomentosus) and additives such as our plain simple lemon juice, Red Sea Aiptasia-X, Salifert Aiptasia Exit, Joes juice aiptasia killer (This is a process where you need to inject the additive in it’s mouth).
So it’s better to avoid all the above situations, it just makes things little more complicated. We were jump away, when we see our tank affected but having multiple tanks in hand it’s lot more easier for us to take charge but in our home aquarium it’s nothing less than a nightmare.