If you are unsure if a rock is a disguised golem or not, a spyglass shows their name and level even when disguised. Also, when disguised, the rocks often move slightly as you get close, re settling onto the terrain.
Unless you are unmounted (rock throw can OHKO unarmored survivors), the Rock Elemental is really easy to run away from, it’s incredibly slow and loses interest in the survivor quickly once you get away a good distance.
Killing one is a bit tricky without a Giga (which ignores the Elemental’s armor). You have a few options however…
1. Bleed. Thylas, Carnos, etc… Bleed ignores HP and armor and can kill the elemental, just be aware you probably won’t get any XP this way unless you kill the Elemental with regular (non bleed) damage from an attack.
2. Explosives. I wouldn’t advise this without a crap ton of resources or a high quality RPG. Elemental’s take full damage from explosives but their massive base HP pool means this will still take a while.
DO NOT USE A TAMED ROCK ELEMENTAL. Like the Giga, tamed Elementals are weaker than their wild counterparts even when saddled. Unless your Elemental is a much higher lvl with a very strong saddle, do not even think about fighting fire with fire.