Cut the top end of a 1/2 PVC tube with a hacksaw to insert a single edge razor so you can reach the glass. For an extra precaution apply a small amount of super glue to hold the razor so it doesn't get loose while working away and you'd find yourself with a magnet tied on with fishing line to the end of the PVC and your looking for the razor. Hope that might help for one suggestion.
If you can't see the glass cause of distortion of the water then two things. First is kind of kooky, build or have a plexi glass type box as some people use the round type on the lens of the camera to view into the tank or shut down any pumps thats distorting the water and scape easy to not disturb your view on the surface.
if you keep a decent magnet cleaner attatched to the glass permanently and get a routine of wiping the glass over daily even if it looks clean you won't get coralline build up.
i know that won't help at this point but should prevent it down the line
Get a magnet mod razor blade they are like 10 bucks then you just clean it like using a magnet just be careful around the silicone. Once you use it once you will wonder how you ever lived without it.
Yes I made mine one Sunday when i was bored. I just sanded the back of the magfloat, bent a piece of acrylic then used some weldon (all i had) with a clamp and I was in business.
I use a small piece of acrylic and a nylon screw with a Wing nut hold the razor in place. I planned on cutting a channel for the razor but I never did.
It was just scrap I had lying around, I really wanted the fancy one from oceanmotions but my DIY one works just as well.