A rock amongst other rocks
Two theories were proposed - either Curiosity accidentally flipped it into its path or it just landed there after a meteorite impact nearby.
Or, in the case of the rather 'imaginative' self-proclaimed astrobiologist Dr. Rhawn Joseph, it is without doubt biological and if Nasa don't investigate it he's going to sue them.
He's already lodged a lawsuit with the US District Court Northern District of California which states:
"Petitioner immediately recognised that bowl-shaped structure, hereafter referred to as Sol 3540,resembling a mushroom-like fungus, a composite organism consisting of colonies of lichen and cyanobacteria, and which on Earth is known as Apothecium.
"When examined by Petitioner the same structure in miniature was clearly visible upon magnification and appears to have just germinated from spores."
Obviously there are problems when zooming in on images and making conclusions about what you see as is helpfully explained by Universe Today here.
But good luck with the lawsuit Mr Joseph...