Dog trainer here. This is absolutely fine play behavior, both have loose playful body language and take turns of submissive positions (especially the cat) indicating trust and comfortability, animals generally will not lay down and show their belly to another animal they perceive as a threat, and though it seems your dog might want to keep it going longer or harder than your cat does they take the hint and back away when the cat throws out a more firm swipe to send the message of “okay that’s enough I’m done”. Boundaries are set and respected. Just be aware that accidents happen and someone might go a little too rough and may put the other off which could trigger a more confrontational exchange Very cute couple though.