Beautiful tank you got there...
It appears to me that you do have enough live rocks to not have any ammonia issue. Nitrates are more difficult to keep in check via filtration without some water changes and/or absorbent media... But I think the live rocks in there alone would be sufficient to digest all traces of ammonia given enough time to balance out.
So having said this, I would suspect overfeeding, or some type of pathogen keeping the bacteria from flourishing. The latter, pathogens... it could be anything like traces left from previous medication or antibiotics used in the tank or on majority of the fish in the past.
I do feel that your tank is a bit overcrowded for a 75 gallon, but not over what your biological filter and live rock can't keep ammonia in check. You also seem to have enough powerheads and pump returns to satisfy water movement.
I'm suspecting overfeeding maybe... try feeding your fish half of what you feed normally for a week. It should not have any effect on fish to do this...and see if the ammonia becomes untraceable low. If it does, then you can add a more powerful protein skimmer and live rock to compensate for the load you have, or simply move them all or in part to a larger or second tank.
If the reduction in food does not reduce ammonia levels, you probably have something in the water preventing bacterial formation. But it seems to me your feeding is reasonable amount for the fish...(unless what you mean by one cube is like half pound of mysis! I am assuming it's a small half-inch cube common to frozen food packaging.
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I will rule out salinity, pH or temperature issues since you look like you have thriving corals... and these parameters don't affect the bacterial colonies too much unless the changes or parameter is way off the chart. If that's the case, you'd have dead corals before anything.
On the clams, since you began keeping them...they need absolute pristine waters and HUGE lighting requirements. So do some research on how to keep them healthy and go get the right equipment needed to keep them alive, especially in the lighting department for the clams.
Lastly...I can't assume this really from the photo...
But, your tank in the photo seems to be a tank that's doing very well from the looks of the yellow polyps and the hard corals extending well. Maybe your tank is still too young with too many changes in the animal loads for the bacteria to have adjusted. Perhaps get another test kit and see if the ammonia levels can be read differently? Could be reading error in the current kit. (like looking at the color of the chart in the wrong lighting. Take it outside and compare the chart and sample in brad daylight, which is always an accurate way to see the color.
Anyway, keep up those water changes and keep the ammonia low till you can get to the bottom of that issue.