How efflorescence forms
The Building America Solution Center's introduction to moisture flow describes the white line often seen a foot or so above the ground on older brick foundations: water wicks up from the ground by capillary action, evaporates at the surface, and leaves behind salts and other minerals. The National Park Service's glossary of masonry deterioration problems calls efflorescence a whitish haze of soluble salts, which can be drawn in from the ground, from road and sidewalk de-icing salt, from fertilizers or from the mortar itself.
A salt deposit isn't automatically harmless. The same NPS glossary says efflorescence often signals that water has found a way into the structure, and that it can point to salts building up under the surface (subflorescence), which can push off the face of the masonry. Treat the residue as evidence of moisture, not as the whole answer.
Why color and texture don't settle it
Mold isn't just black spots. EPA's Mold Image Library notes that molds come in many colors, forms and textures, and that the same species can change color and appearance over time. Going the other way, EPA's mold course chapter on finding mold and moisture warns that not everything that looks like mold is mold: alkaline crystals on soil or concrete walls can look like mold, though unlike mold they're usually water-soluble, and paint or stains can fool you too.
So describe what you see rather than labeling it. "White fuzzy deposit along the bottom of the brick wall, first noticed in March" is more useful to whoever looks at it than "harmless salts" or "toxic mold," and dated photos of the same spot help show whether it's changing.
Look for the moisture behind it
Whatever the deposit turns out to be, the moisture is the real problem. Note where it shows up, whether it gets worse after rain or snowmelt, and whether the area smells musty; EPA's course says musty odors should alert an investigator to possible mold, and that mold can grow in hidden places like the back of drywall. If the wall is in a basement, our guide to documenting basement water before a repair walks through what to record, and the comparison of interior drainage and exterior waterproofing explains the main ways water gets handled once its path is known.
Testing, and what not to do yourself
EPA's brief guide to mold, moisture and your home says that in most cases, if mold growth is visible, sampling isn't necessary, and that hidden mold can be hard to investigate because disturbing it can release spores; it suggests an experienced professional if you think you have a hidden problem. The mold course adds that sampling results are hard to interpret without experience.
Don't scrape, chemically treat or open up a wall as a way to identify the deposit. EPA's brief guide also says not to paint or caulk over moldy surfaces, since paint applied over mold is likely to peel. If humidity rather than a leak seems to be the issue, our sump pump versus dehumidifier guide covers what each one actually controls.
Before anyone treats it
Ask whoever assesses the wall what supports their identification, what's still uncertain, and where they think the water is coming from. A good answer separates what they observed from what they concluded.
EPA's brief guide is blunt about mold: if you clean it up but don't fix the water problem, it will most likely come back. Efflorescence works the same way, since it keeps forming as long as water keeps carrying salts to the surface.