What is the ruling on the aforementioned supplication: "My Lord, just as You nourished my body when I was a fetus in the world of fetuses with the placenta's nourishment, when I was needy and devoted to You, my Lord, do not cut off Your sustenance from my soul in the world of souls, for my body has grown large, but what can I do with this small soul?"?

It is more complete for a Muslim to supplicate with the ma'thur (reported) supplications from the Prophet, peace and blessings be upon him, as they combine the good of this world and the Hereafter. However, there is no harm for him to supplicate with whatever he wishes. This particular supplication, about which the question was asked, does not have any specific merit that we know of, nor does it appear to contain anything prohibited. It falls under the category of seeking closeness to Allah through His actions, which is permissible. We have found this supplication widespread on the websites of some people of innovation.