This is perfectly safe, Timothy hay is actually a type of grass hay. Also Timothy hay is a good hay for adult rabbits.

I cant say what that plant is, you could perhaps ask the farmer? Farm hay is often a mix of grasses/plants/weed with one predominant grass type (ie timothy, orchard, fescue etc). 

I inserted the picture into your post so it’s simpler for people to see it. Maybe someone else recognizes this plant.

I actually have seen this in my Timothy hay a few times in the past. I too was perplexed and I thought it was not grass hay so I first complained to the company that there was some foreign plants in my hay order… they kindly replaced it (there was a lot of other weird unidentifiables in it so for now I won’t feel so bad, better safe than sorry) but after I began seeing it twice and more, I have now kind of concluded that they are a normal part of Timothy hay from time to time… With that said, I still pick them out and if any get past me, my BunBun won’t eat them anyway.