Your options choking from the mount are a bit limited in no gi. Other than the arm triangle, here are two I've seen done.
1) Guillotine. Exactly the same as from guard, but imagine that you have swept your opponent into the mount. A lot of places will consider this a neck crank, but it can be a choke too depending on how you do it. I don't usually go for this one myself because it leaves you vulnerable to being bridged. However, if you sink it from mount, the bridge shouldn't matter because you just finish from the guard.
2) Forearm drop. This isn't really a choke that taps people; it is just used to create other openings. Just make a fist and place the blade of your forearm over the side/front of your opponent's neck. Aim for the same place you apply pressure to in the RNC. Then instead of just pushing with your arm, imagine locking your arm in place and dropping your whole body weight through your arm. You can't actually tap anyone decent with a move like this, but even the best guys will reach up with their arms to defend. Then you can go for an arm triangle choke from there or any other move you feel like.