Hello Long. This is a type of string or beard algae. I've had it before in a couple of my tanks and use a long toothbrush to remove it. It's best to start removing it as soon as you see it or it will grow to the point you'll need to remove everything from the tank that it has attached itself to and put the pieces outside where they'll get full sun. The algae will dry up in a day or two and make removal easy. But, then you have to rescape the tank. I've done that too.
10 Tanks (Now 11)