Hey mama. Neck rashes (or rashes in the folds of the skin) are very common with infants. As a pharmacist, I’ve seen this often in little ones due to excessive moisture/sweat in the folds of the skin. Sometimes it could be fungal. I’d definitely get it checked by a doctor if you have the opportunity too. Leaving a fungal infection untreated can lead to severe irritation. In the meantime, try keeping the area clean and dry if you can.
I would definitely go see a gp about it. Hope it gets better soon!