You have a tough one here. I looked at the actual census microfilm, and confirmed what I thought might be true. Silas Clark was living at the Lorain County Infirmary in Carlisle Township at the time of the census, not with the family of Isias Bruce.
There is an entry line after the Bruce family enumeration that reads "Residents of County Infirmary", then the listing of residents, including Silas Clark. The fact that he was living at the infirmary in 1880 would indicate that he was without means, and may have had no family in the area that he could live with.
Next week I could easily check the 1860 and 1870 census indexes for Lorain County, to see if he was living in Lorain County prior to moving to the Infirmary. He was probably living in either Lorain County, or an adjacent County during this time. Do you have the names of any family members of the Silas Clark family you are researching? There were a couple of Silas Clark families living in adjacent Cuyahoga County in 1850.