What type of spiders eat plants--I have a small brown fast spider on my Bonsai tree. I need to know if it's okay to leave him there?

Have a look here, usually its the red ones that are harmful because you can see the damage

http://www.bonsaitreeforums.com/forums/bonsai-trees-forum/11271-red-spider-mites.html

http://ezinearticles.com/?Spider-Mites-And-Your-Bonsai-Tree&id=620709