Wont come off?

Hi Guys,
These spots have been gradually appearing on the back wall of my tank for some time now. I've tried scrubbing and scraping it off, but it wont budge. It seems its easier to scratch the plastic the back is made of then get this stuff off. The spots are a yellowish green/brown.
Their kinda getting out of control. What do you guys think?

look like algae or coraline growth, not quite sure what to tell you if scraping wont get them off

That's what is hard about the biocubes having plastic backs. I had one for a while and it drove me nuts as you cannot use the regular mag floats or razor scrapers on the plastic. The back of mine ended up with a lot of that green and purple stuff, I tried the rough side of a sponge and went to town on it, ended up causing lots of scratches on that black plastic (I did the same thing on my jeep paint job, idiot) I think most people with biocubes just let the coraline and algae take over the back.

I agree with Hannah, looks like coraline growth...

That's what I figured, hopefully there will be lots of nice colors back there.