What causes a white outgrowth on the sclera?

Scleral growth: Your description could quite a few different things. The most common would be a pinguecula which is a benign growth related to uv exposure. It could also be a precancerous or cancerous lesion so you need to have the lesion evaluated by an eye doctor.

Probable pterygium : A growth that may occur from a "clock" perspective at either 9 or 3 o'clock positions on the white part of eye with growth towards & onto clear part is called a pterygium.Often caused by excessive sunlight exposure, dryness, wind, dust, excessive outdoor activity without uv protective glasses, and possibly from other adverse conditions that cause chronic eye irritations. Other: pingueculum, tumor.