Most pool growths are heterogeneous, and encompass both bacteria and algae. Virtually all slime growths are like this. Terms like "green algae", "mustard algae", "black algae", etc. describe typical behavior of several types of organisms working together, rather than a homogeneous colony made up of a single type of bacteria or algae. A researcher I knew at Buckman Labs (originator of the polyquat algaecide) did some studies on "algae" and "slime" he pulled from his own home pool. My recollection is that the FEWEST different organisms he found in any sample was more than 20!

But . . . black/dark gray sludge/slime often indicates the presence of anaerobic bacteria (bacteria that grow in the absence of oxygen). This in turn indicates a lack of circulation in the area where such sludge appears. Some of the bacteria that can form in such situations are HAZARDOUS to pool users.

So . . . clean out the niche. Wipe it out with polyquat, if you like, as a sort of 'inoculation'. Make sure any water vents around the lamp are clear and open, so flow can enter. Chlorinate your pool heavily -- see the "Best Guess" chart for appropriate shock levels, and then maintain adequate chlorination all the time.