Johnny Manziel -- HOLDING BRICK OF CASH ... Pretends It's Cell Phone

Here's a brand new video of Johnny Manziel that's blowing up on the 'net -- showing the Browns QB clutching a brick of cash ... and talking to it like a cell phone.

Why? Why not.

The video first surfaced on Instagram over the weekend. Unclear exactly when and where the video was shot -- but Johnny sure seems be having a good time.