I just noticed that the metal part of my Philips sterilizer is rusty. anybody else has this issue? I've only used it for 3 months. is it harmful for baby?

What you are seeing is limescale deposit. It builds up in appliances when the residue of minerals (naturally contained in water) fail to evaporate when water is heated. Though it is not harmful, it is unsightly, hard to clean and may impair the operation of appliances or damage components if allowed to build up. You can take a look at this document by Philips for instructions on how to do the descaling. https://images-eu.ssl-images-amazon.com/images/I/91wZosFU4sS.pdf