If you mean shorting Masimo purely to drive down the price, that's market manipulation and is definitely illegal. E.g.
https://www.investor.gov/introduction-investing/investing-ba...
https://www.investor.gov/introduction-investing/investing-ba...
https://en.wikipedia.org/wiki/Hindenburg_Research would be out of business if that business model was illegal. You can short all you like; you can't be deceptive/fraudulent about it.
Hindenburg compiles and releases information they believe to be truthful about a company, which they shorted beforehand in order to profit from the information.
What I am talking about is sending orders only to cause a certain reaction (e.g. by convincing momentum traders that there is momentum in a certain direction).