I can play harmonium and now, I want to learn piano. Both are keyboard instruments. Are both the instruments played in the same way or do I need to learn something more to play a piano?
Both are KEYBOARD instruments.
I play a tracker organ, acoustic piano, electronic organ and electric piano every week for various jobs and I play them exactly the same, from the weight of my arm. Arm weight equalizes everything. On the organ I leave just enough weight to "rest up" yet sustain the tone but really, it is not much different than playing legato without using the sustain pedal.
Using in/out, up/down, the pronator/supinator muscles and gravity, those movements takes the burden off the long flexors and playing is effortless. Well, unless you abduct or have a wrist deviation.
When people complain that the action may be stiff or shallow on any particular instrument it is because they are trying to play from the fingers (which don't have any muscles) and not the arm. It could also be that they have some improper movements such as abduction, ulnar or radial deviation which is getting in the way of the arm's alignment.
Piano playing is as much an athletic sport as any other. There are laws of physics and bodily ergonomics that must be adhered to. Break those laws and there will be an orthopedic surgeon in your future. If you are lucky, only mediocrity.
If you have ever taken golf lessons, you know about hand and finger placement, balancing on the balls of your feet, alignment, equal and opposite motions, rotation of the shoulders and hips. It isn't the hand that strikes the ball with the club, it is the whole body and the hands are the conduit. Playing any keyboard instrument is much the same. People who don't know this will disagree but that is okay. There are many roads to the same destination but often it is what we don't know about anatomy that holds us back.
Playing should be effortless and if it is not, there is a movement or alignment problem somewhere.
Piano most certainly is not a reed instrument but a percussive string instrument. "Piano" is short for "pianoforte" and means that the instrument will sound with different loudness depending on how hard you strike the key. The tone will decay on its own with a comparatively long sustain or it can be cut off by releasing the key (unless you use the sustain pedal). A harmonium gets its sound energy from the bellows, a piano from the hammers striking strings with the strength resulting from the force with which you strike a key.
In short: the keyboard looks similar but the instruments are quite different in playing, sound, articulation. There is no point in striking the keys of a harmonium forcefully or with graduated strength. With a piano, it is the main means of expression.
Portable harmoniums are often played with one hand while the other pumps the bellows. Obviously, this isn't necessary on piano so you would have both hands available at all times.