What causes algae to grow on the gravel and sides of tank?

Too much light, for too many hours, will definitely grow algae.

The newer lights utilized to often will produce this result

A lot actually contributes to algae to grow . basics are temperature, lightining, light cycle, waste etc. One big contributor is there direct sunlight on the tank? Without any of the pertinent information like parameters location lighting this really can't be answered without knowing these factors

Too much light

Best way to get rid of it is adding a lot of live pants. The plants will soak up all the nutrients algae need before it can and your fish will love swimming around them too

Generally overfeeding and/or under cleaning. Direct sun light can have a large impact too.

Could be a variety of factors from too much light, overfeeding, bad water quality. What color is the algae?

Excessive nutrients in the water and could be the lights are on too long

Bacteria in the water/algae tablets/etc.