u/Braveplanet

I recently purchased a new to me vehicle, I'm super happy except for this white-blue stain behind the seats.

Lefthand side is more white and flat like something soaked through Righthand side is more of a blueish build up

I'm not sure what it is... Salt? Battery acid?

I wanted to see what everyone's recommendation on tools and solutions. Should I go in with a rag soaked in hot water?
I don't have a steam cleaner but have one of those Bissell little green carpet cleaners.

I don't know what substance is so don't want to make it worse. I appreciate everyone's feedback and suggestions!