Hi
This little one is cute! The spots don't look like a disease or insects at the moment, more like a little environmental stress. Check two things:
The pH of your water (ideally between 6.0 and 6.5).
The humidity (52% is a bit dry for this stage, it would like to rise to around 65-70% if possible).
Keep it up, the first leaves are rarely perfect, it’s the new growth in the center that matters most!”
My personal opinion:
The beginner's reflex is often to want to feed the plant as soon as we see a small spot. Here, the plant is still very young. I would tell him to stay in clean water for a few more days to see if the new leaves come out green and without spots before bringing out the big guns!
I hope this will help you. good luck my friend