I'm wondering what is this white splodge on my back molar?

Cavity. It's a cavity stating from either bad oral hygiene , bad diet or too much flourid like in drinking water for example. Get it treated asap before it turns to othe thing worse to treat later.
White spot. White spots are usually either malformed tooth or demineralized tooth which is an early stage cavity. They most often come from too much Fluoride or poor oral hygiene. Hopefully changes to diet and work to alkalize mouth and saturate saliva with minerals could stabilize spot. Sometimes they can need fillings.