re: Do you call it brush hog or slashing?

You are correct. Brush hogging, or underbrushing, is the term used for heavy brush and small trees.

Slashing is a new one on me but I see where they're coming from as the term slash is used for the mess left over at a log landing when a pine stand is clear cut.

Bush-hogging is generally reserved for heavy grass and weeds and maybe light brush and transplant trees about one inch diameter or so.

I'm sure areas differ a little but these are the terms I've heard used for my last 37 years running tractor bush hogs, skid steer mulchers, dozers, excavators and breaking ploughs.