Betta Fish: What is this weird red stuff?

I was replacing some of the water in my sisters tank yesterday and noticed what looks like a weird red growth on the filter? I noticed this a couple weeks ago when replacing the filter but it was a lot smaller and looked green so I assumed it might’ve been algae. But has gotten bigger and red. The fish seems fine and going to do some water tests later to make sure everything’s fine, but wanted to make sure if I should be trying to scrape this off or if this is normal? We are new to maintaining a tank and betta so any incite would be appreciated!