Whitish brown spots on leaves.

Hi,
First time grower here, I'm 5 weeks into flowering and suddenly I noticed these spots on the plant leaves and I have no idea what it is. I asked a couple of growers in my State and they said I have a mag deficiency in my plants. Please help me to identify the problem.

Here's what I fed to this plant last week.

CalMag 0.5 grams in 2 litters of water ( pH 6-6.4)
And 0.25 ml fulvic acid.

Thanks.
Smith