Using Agar agar beyond its use by date
If you store it properly (i.e., dry), it won't go bad in the unsafe sense. It has a pretty long shelf-life. Its possible that it'll lose some gelling strength eventually, particularly if stored in harsh conditions (elevated temperature, exposure to humid air, exposed to sunlight, etc.). Like anything else, it could conceivably pick up off-flavors if not stored in an air-tight container.

I'd make a small bit of agar-agar gel with it, to see if its still OK. I expect it is.