I actually used a diluted mixture of muriatic acid and water to wash the coraline. It took a lot of elbow grease and scrubbing, but I was able to get most of it off. I also used a razor blade where I could. But I didn't want to scrape up the overflow, so I was careful with it. Then I rinsed the heck out of it several times to make sure I got it all out.
Muriatic acid is reef safe, so I know it's freshwater safe too. I wash all my pumps in a diluted muriatic acid mix. It's actually cheaper than vinegar in the long run cause you dilute it to apx 1:10 (acid:water) mix. You can pick it up in any Home Depot gardening section for like 9 bucks.
Be careful with it though. Make sure that you add the acid to the water... and not the other way around. I also wear pvc gloves. If you have an open cut, it will burn.