Oh coralline how we long for you, love you and hate you...lol
This is a funny part of the hobby how we so want the corraline to grow faster and take off then when it does all we do is spend time scraping it off... There are many ways to do this and acrylic is the tricky one:
1. Keep it clean so you don't have to scrape it. Use a mag cleaner to keep up on maintenance so it doesn't get out of hand. Add this to your daily maintenance routine.
2. Dobi pad or Magic Eraser (unscented) can be used to take off the corraline that the mag float cannot. You can cut a slice of the pad off and use the mag float to scrub the glass.
3. Plastic, acrylic safe scraper for the hard stuff that has been left to grow for awhile. This is the most harsh and should only be used of the first two are unable to get it off.
Never use a metal tool of any kind to scrape the acrylic. That is almost guaranteed to scratch the delicate surface of the tank.