Hey I got those in my tank about 2 months after setting it up. They were very annoying. I got tired of cleaning them knowing they would come back the next day.
Here's how i fixed it.
Make sure the tank is cycled
Scrub as many off as possible.
Clean the gravel and walls.
Do a 50% water change.
Remove activated carbon from filter and add tetra algae control (was about four dollars at walmart).
I also added more light as some say low light can help them prosper.
Repeat the algae control if necessary.
By now the diatoms were all grey and dying, i scrubbed them, did a water change, and cleaned the filter.
I also cut down on plant fertilizer and water clarifiers, not sure if you use any. They are pretty much gone now. Just showing you what worked for me, i'm not sure if it will work for others.