Mr. Heckles, played by Larry Hankin, is the character I assume you are referring to. He lives below the girls until he dies in the Season 2 episode "The One Where Mr. Heckles Dies". He has wild white hair, and is often seen wearing a brown robe. He is seen in five episodes of Friends:
"Friends" .... Mr. Heckles / ... (5 episodes, 1994-1996)
- The One with the Flashback (1996) TV episode .... Mr. Heckles
- The One Where Heckles Dies (1995) TV episode .... Mr. Heckles
- The One Where the Monkey Gets Away (1995) TV episode .... Mr. Heckles
- The One with Two Parts: Part 1 (1995) TV episode .... Mr. Heckles
- The One with the Blackout (1994) TV episode .... Weird Mr. Heckles
http://www.youtube.com/watch?v=cGVq95gWGeA
If you'd like to know more, go to www.imdb.com