Hi, welcome to TONMO.com!
We just discussed this very situation a few days ago - look at the thread listed below. The person who found the baby octo did not have a tank set up, so he ended up passing the baby to someone who was better equipped to take care of it.
http://www.tonmo.com/phpBB/viewtopic.php?t=2511
How big is the baby - tiny, the size of a pea? You need to keep it in a smaller container so you can feed it frequently. Try mysid shrimp or amphipods.
Perhaps you could keep it in a smaller container floating in a larger tank, or just in the larger tank (like a critter cage).
Under Ceph Care, Journals and Photos forum, you'll find some threads on Imahogg, a baby briareus. You can read there how it's being raised.
We welcome your future questions over in Ceph Care! Good luck!
Nancy