Slime Mould vs Mould: What Is Physarum and Is It Harmful?

Physarum polycephalum is a slime mould, not a fungal mould. That difference matters because active Physarum is a mobile plasmodium that can spread as a sheet, fan or vein-like network, while fungal mould usually grows as fuzzy, powdery or filamentous hyphae.

If you are trying to tell the two apart, the most useful clue is change over time. Physarum often shifts position and shape as it feeds, while fungal mould tends to expand in a more static, colony-like way. A single photo can be misleading, especially if the culture is dry, resting or partly obscured by food and substrate.

Active Physarum may look yellow, thin, dense or branching depending on light, moisture, food and growth stage. It can also form a hardened resting structure called a sclerotium when conditions become less favourable. That resting form is not the same as an actively feeding plasmodium, and appearance alone cannot always confirm viability.

For a live slime mould culture, the practical question is usually not just “what is it?” but “is it active?” If you want a broader care overview after identification, see the slime mould care guide. If you are comparing a culture with springtail activity, the article on do springtails eat slime mould may also help.

Physarum is not the same thing as fungal mould on damp surfaces or food. If you see fuzzy growth, that is more consistent with contamination than with active plasmodium. When the culture looks thin, dry or inactive, compare photos over time and check moisture, food condition and recent movement before drawing conclusions.

In short, Physarum is a living slime mould with a mobile feeding stage, while fungal mould is a different kind of organism altogether. If you are choosing between a live slime mould culture and a general mould problem, the deciding factor is usually whether the growth is moving, networking and responding like Physarum rather than behaving like a fungal colony.