Carl,
If the seacock is seized up you'd probably be better served by making sure the hose clamps, hoses, etc of that intake are in good condition than attempting to close it. When a seacock seizes up it can sometimes be a real pain to close and sometimes owners will break the seacock in the process of its closing.
Its likely this seacock has not been operated in some time. Wait for the next haulout and then disassemble it properly and service it. After you've gotten it back into good working order make sure to operate it often so that it will stay lubricated.
I personally wouldn't worry too much about it unless you see something actually wrong. Its one of those nagging boat problems. I had the same situation. I replaced the hose clamps and waited to haul out to correct it. Not optimal, but better than breaking it.
Just my .02c
Good luck!