I'm on my first grow but had something similar. Searched high and low for similar cases but couldn't find anything. Small dots on old growth. A huge increase of them, then they joined and started looking likes cracks. If iti is actually the same thing, I'm afriad I can't tell you what it is, but in my case, the issue was caused by the growing medium ph being incorrect. I corrected the ph with a good flush and halved my ppm for the first day or two and growth has returned completely to normal.
I, however, could say the next most likely cause in my case, is root issues. I noticed a lot of dead roots at the bottom of my rockwool blocks during this time - I attributed it to one day too many without water, so I'm also not certain about that.
Also, I could very well be wrong, as I don't have that much experience.