λαμβάνεται is middle, “is taking hold of,” and the middle of λαμβάνω routinely takes the genitive. (It’s a “partitive” genitive—he doesn’t take the whole altar. Similarly θιγγάνω “touch” also takes genitive.)
2, μὴ ποίει μηδέν “Don’t do anything” (lit. Don’t do nothing), ποίει imperative (note the accent). In commands the negative is always μή not οὐ.