We just introduced a new puppy into the house (a Yorkie Poo, very tiny), and I'm having trouble telling if my other dog (been in house for a few years, a Miniature Schnauzer) is being aggressive towards it or not. My puppy (the Yorkie) wants to play and chases my Schnauzer around (slowly, as she's a puppy), and my Schnauzer avoids her at all costs, running away constantly. However, today the Schnauzer put the front half of his body down and the back half up. I know this means he's playing, but he was making almost like a hissing noise and would smack his front paws on the ground. He barked a little bit, so I'm really unsure if I should be seperating them when this happens or to just let them 'play' (if that's what they're doing)?

P.S. The schnauzer is around 4 years and a male. The Yorkie is 6 weeks and a female.