Although your silicone piece is incredibly flexible and strong, this doesn't mean it's unvulnerable. It can tear under extreme stress, e.g. when pulled way beyond it's natural shape.
Over time, with use, the silicone will lose its shape; this is unavoidable. To prolong the shape, store your mould in a cool environement it's own, with nothing crushing it from any side.
Please note that with the mould curving around the top of the plug, it will be thinner in this area only. Take great care to ensure the mould has separated from your plug before you pull it out as it can tear.
To clean, use warm water and mild soap only. Allow to air dry for several hours before its next use.