[piercho]

Stumped for an ID, but I like the narrative. Brown, red, and green algae can take a brown color so its hard to go by color. Of the algae common to reef tanks Ulva, a green algae, is sort of similar but it ususally grows near the waterline. Its not unusual for algae to grow in clear or opaque pipes. Its one reason the thicker, darker, schedual 80 PVC is used instead of the white schedual 40 PVC which allows quite a bit of light through. I used black cellular ABS (sold as drain pipe) for piping and for skimmer reaction chambers which allows no light through, but its not available in diameters smaller than 1 1/2".