What is the significance of the white band that appeared on Dr. Throckmartin's chest?

The white band on Dr. Throckmartin's chest is a distinctive mark, described as being 'white as pearl' and 'about two inches wide', circling him above his heart[1]. It felt 'cold--like frozen marble' and could not be burned or marked[1][1].

Throckmartin states this band is a 'brand' or 'mark' put upon him by the 'sparkling devil' or 'Dweller' he encountered on the moon path[1][1]. It gleamed and writhed under moonlight, and he believed it was 'coming--to take me back to Edith!'[1][1]. The mark's size fluctuated, becoming 'smaller' when he felt he was escaping[1].

Space: The Moon Pool by A. Merritt