Jungle Giving Way to Savanna
Your orcs share many similarities to humans in that they are both bipedal, roughly the same size and have evolved very high intelligence (for the animal kingdom). So therefore the pressures that made humans will therefore be similar to the ones that made orcs, as this is how convergent evolution works.
The Orcs ancestors were like those of humans; tree dwelling apes that needed to adapt to a changing climate that opened up grasslands. The proto-orcs got their humanoid build and intelligence from these analogous environmental conditions.
But there are notable differences.
The tusks of orcs evolved from lower canines that didn’t really serve a particular purpose, but were not harmful. Over time it was selected for sexually much like the antlers of a caribou, which is why they are more pronounced in males but not absent in females.
A Lusher Grassland
The savanna that the orcs evolved in was similar to man’s cradle in the Great Rift Valley, but with some notable differences. The rains were more frequent, leading to taller and more verdant grasses that limited endurance hunting like humans perfected. Therefore orcs evolved a greater sense of smell, in order to detect creatures hiding in the grass as it moved about, and to smell blood, as it couldn’t rely purely on good vision like early human hunters.
An unfortunate reality of the verdant plains for our proto-orcs was the presence of dangerous predators. Orcs evolved their greater strength, better smell and penchant for violence due to the presence of large predators that they needed to fight off at any given moment.
This is the only part that’s tricky. Although there are mammals with algae in their fur such as sloths, no mammals have it in their skin and they definitely don’t provide energy. To have an animal take advantage of photosynthesis in its body is an incredible and one in a billion adaptation that is highly advantageous. Orcs got lucky and now get a little more energy from the sun via the microorganism’s photosynthesis producing sugars that the orc can absorb.