Defiantly a mag deficiency, why is what you need to figure out, PH issue, something else blocking the mag? Sadly those leaves wont get better to tell you and there probably isnt a lot of new growth going to pop out and tell you, but keep an eye out for it.
Let us know what your feeding it, what order you mix the nutes in, and what is your PH
To start I am not much of a soil grower, but I would flush it with plain water at 6.5 or a little higher and then start feeding again at 6.5 with cal/mag. In Flower I like to use botanicare cal/mag plus because it has less N.