Why is my coralline algae falling off the back glass?

What type of lighting do you have? If not LED, when did you last change the bulbs? PC, MH and T5 lamps lose their color spectrum and PAR long before they burn out.

 

Aquatic life 1w led's - had them over just over a year now

 

With the lights off, take a close look at the diodes. With some cheaper/older LED units, especially those with 1w LEDs, the diodes become discolored over time, which dramatically affects the spectrum (Marineland LEDs have reportedly turned black in 12 - 18 months). One wouldn't always notice by looking at the tank.

 

Thanks for the advice! So when looking at the diodes, what should I see (if they are good) or what possibley would I see if they are bad, would they turn black?

 

They may be black or have a foggy appearance if they're bad. Otherwise, they should appear as clean colored glass.