There are no examples of persons named Slonceida in the records found on Ancestry Library. There is one immigrant woman listed as Slonce in 1920 who seems to be "Sally" in the 1930 census.
The only listings on Google for Slonceida besides on baby name sites seem to be its use for a character named Slonceida Sol in an unpublished graphic novel being written by a blogger called "deepdive" whose real name seems to be KarenGunter:
At the moment Slonceida seems to just be a name probably created by and certainly suggested by "Alexander Nevzorov". If Ms. Gunter is ever able to publish her novel, there might be real children given the name, but at the moment there's no evidence for its use for a non-fictional human being.
P.S. Although it's not 100% clear in other places that Mr. Nevzorov created Slonceida and his other suggested unusual names on his own, I found the following question he asked on a board about Roman Catholic names where he clearly says that he invented them: