Diatoms look brownish and sometimes dusty, covering surfaces of the tank, even the glass. Do you have a picture of it?
It's pointless advising anything if it's not diatoms, could be something else.
Don't point the filter downwards as the tank needs surface movement for oxygen.
However, the filter may not be strong enough to spread the flow all around the tank. I second filter will help, especially that tanks like that are normally overstocked to work well with the cichlids.
What's the intensity of the light, as in Watts and how big is the tank?
Diatoms are common in new, unestablished tanks and once the filter starts coping with the biload, they stop appearing. But that's not the case if the filter isn't large enough to cope with them.