Is it permissible to greet with "Salam" someone entering the mosque while I am seated, taking into account the noble Hadith: "He is not of us who does not revere our elders"?

We found no difference in the ruling regarding a seated person standing for a standing person, whether inside or outside the mosque.

It appears there is no harm in standing for the aforementioned individuals if it is done out of kindness, honor, and respect. Al-Ayni, in his commentary on Al-Bukhari, mentioned four categories for standing: forbidden (for one who is arrogant), disliked (for one who is not arrogant but fears resembling tyrants), permissible (as a form of kindness and honor for one who does not seek it and is safe from resembling tyrants), and recommended (for one returning from a journey, out of joy for their return, or for one who has received a new blessing or experienced a calamity). Al-Ghazali said: "Standing out of veneration is disliked, but standing out of honor is not disliked."