Halgerda formosa has following distinctive characteristics :
- the whitish grey body with reticulate pattern consisting in ridges lined with yellow strokes
- the presence of tiny white pustules near the mantle edge
- the presence of round black spots on the mantle. These spots can be variable in number

Halgerda formosa can be mistaken for Halgerda punctata, Halgerda toliara, Halgerda dichromis and Halgerda wasinensis. Here are main differences between these species :
- Halgerda punctata has reticulate pattern whose ridges are indistinct and lined with yellow tubercles.
- Halgerda toliara hasn't any dark spot on the mantle.
- Halgerda dichromis has deeper 'valleys' and elongated black spots.
- Halgerda wasinensis has very dark 'valleys' and numerous small black spots on the mantle edge.

African Indian Ocean