Q.Ficus tree
My Ficus tree has started to have lots of little black straight lines on some of the leaves.

Leaf Spot fungus can leave black spots on the leaves of the plant.
The black lines that you describe could also be a build up of salts in the soil.
This can happen from watering with conditioned water.
Here are some links with more information.
https://www.gardeningknowhow.com/houseplants/hpgen/leaching-indoor-plants.htm
https://www.gardeningknowhow.com/plant-problems/disease/plant-leaf-spots.htm
https://www.gardeningknowhow.com/houseplants/ficus/ficus-tree-care.htm