I have some blix that's been in those plastic collapsible bottles. Recently when mixing up new chems, I noticed that the interior of the blix bottle is coated with a grey layer. I can rub it off with a finger, but not getting more from shaking the bottle a drop of soap.

Is it correct that this is silver? Would agitation with plain fixer do the job?

For reference, this bottle has only been used for E-6 processing.