A mark of gain!
There's not a lot you can do, in case you look to grow in relatively small time period. And even in longer period, it's quite common that stretch marks can appear.
You can moisturise the skin with coconut oil or other variants, but some marks might be unavoidable if you're looking to grow. I have a straight mark line on the under side of my penis, it's reassuring to know that something has happened when I have doubts.
So, choose; to grow and have some marks (even bodybuilders get marks) or stop and go home. I definitely don't suggest the latter one!
Oh, one more "solution" that hasn't been studied in the Brotherhood that much, but was spoken about a longer time ago; derma needling. That's supposed to reduce the stretch mark signs, as it pokes tiny holes in your skin, activating collagen production and so on.
I haven't tried it, but it's a possibility. It isn't an over night process though, you do the needling, apply Vitamin C or other supplement creams and in about 1-2 months there can be a difference. I've only done my face a little bit, I haven't stuck to the regime, but there's some improvement.