Moss, algae, and dark streaks on a roof

Usually not. Flat dark streaking running down the slope is generally algae and is largely a cosmetic issue. Not a sign of failure.

Yes, unlike algae. Moss holds moisture against the covering, works under and lifts shingle edges, and prevents wood roofs from drying. In freeze-thaw conditions the water it holds expands as it freezes.

No. High pressure strips granules from asphalt, erodes cedar, drives water up under the covering, and can affect manufacturer coverage. Ask what gentler method is proposed instead.

Only if the conditions that produced it remain. Shade and slow drying are the cause, so cutting back overhanging branches and keeping the roof clear of debris is what actually reduces recurrence.

They can reduce regrowth on the area washed by runoff from them, which means coverage is partial and depends on placement and rainfall. Treat them as a partial measure. Not a cure, and not as a substitute for addressing shade.

Not by itself. It means the covering underneath deserves inspection, because heavy long-standing moss can conceal deterioration and lifted edges. What the inspection finds decides the question, not the moss.

Less commonly than on asphalt or cedar, since metal sheds water and dries faster, but it can still establish in shaded areas, valleys, or where debris accumulates and holds moisture against a seam or panel edge. The same shade-and-slow-drying cause applies regardless of material.

It is generally sensible to address heavy moss before freeze-thaw cycling begins, since moss holding water that then freezes and expands accelerates whatever damage is already occurring. That said, growth found in winter does not need to wait until spring if it is causing active problems.

Algae staining tends to show more visibly on lighter-colored shingles simply because the contrast is greater, though the algae itself can grow on any color. Algae-resistant granule formulations, where used, target the growth. Not the visible color.

The same shade and moisture conditions that grow moss on a roof can support it on adjacent siding, trim, or fencing, so seeing it in one place is worth checking nearby surfaces too. Not assuming it is confined to the roof.

Generally yes: lichen bonds tightly to the roofing surface and can pull granules or surface material with it when removed, which is why it requires gentler, more deliberate methods than moss, which sits more loosely on the surface.

It substantially reduces the odds by restoring drying conditions, but it is not an absolute guarantee: a roof plane with unavoidable shade from a building, a north-facing orientation, or nearby structures can still be prone to regrowth even after tree work.