What is it? It grows..

[pavel]

Has grown itself. It is interesting to me, that this such. On a photo beside a tube of 16 mm.

[Dave66]

It's hair algae, my friend. Though it's green, it's a red algae; it's using chlorophyll in concert with its natural photosynthetic compound. I paraphrase something I wrote about it in a piece I wrote about solving algae problems.

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BLACK BRUSH ALGAE

The pirate that is Black Beard or black brush algae isn't simply green algae run amok. It's a red algae of the Genus Audouinella and is totally a different kind of creature than the green algae we know.

It almost always occurs either on the edges of live or plastic plants or in the flow of a power filter. The colors can be dark green, dark red or black.

Removing it manually isn't the answer, because at the base of the hairs are spores, thus if you are careless in removing it, hundreds more colonies will appear.

The answer is Carbon Dioxide. If you've pressurized Co2 in a planted tank, raise the level to 30 ppm. If unplanted or with plastic plants, take out the decorations and rinse them under a strongly running tap.
If a planted tank without pressurized Co2 or one that the offending algae returns to even after the rinsing, Flourish Excel will kill the algae. Simply follow the directions on the bottle and start with daily half-doses, as full doses can damage the fish as well as the algae in un-planted tanks.

You'll see the algae diminish nearly daily. When it's gone, it's usually gone for good.

As an aside, tanks harboring Siamese Algae Eaters make such short work of any brush algae that appears it never gets a foothold in their tanks. SAE's (Crossocheilus siamensis) are the only fish I know of that eats it.
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Use half-doses of the Excel, as you don't want to stress your fish with full dozes. Check the function of the product to know why.

Dave