It is a grey-black crystalline solid semi-metal with a sphalerite structure.

[1] Along with other II-VI compounds, colloidal nanocrystals of HgSe can be formed.

Toxic hydrogen selenide fumes can be evolved on exposure to acids.

HgSe is forms large insoluble clusters with proteins during digestion, and a very precise co-administration of selenium during mercury ingestion has shown to reduce the resulting intoxication.

The effect is too finicky for any practical use, but selenium's ability to complex mercury has been proposed to explain why relatively high mercury levels do not intoxicate deep-sea fish.