Stunted?

I see that you mix is not 100% coco, but coco in general can be tricky. If you mixed it yourself, coco can start with salts and have cal-mag buffering issues. This can appear as early tip burn. In coco you need to worry more about it drying out as thats when it can be damaging, but in a soil blend you have to balance that with not overwatering. I would keep a ph of 6.2 at this stage. I would hold off on the rhino skin for a few weeks, I believe that is potassium silicate/liquid silica additive. Very helpful for strong leaves and stems during veg, but can be rough on seedlings and cause swings in ph. Keep the EC under .7 at this stage as well. Overall the seedling looks ok, so if it's growing steadily don't worry too much if it's a tiny bit behind its peers. Happy growing!