Hamza:
Root cause analysis is a process. There are a number of RCA techniques, such as 5-Whys, Fault-tree analysis, failure modes and effects analysis. A cause-effect diagram is a way to map or display the results of a root cause analysis. Examples include a BowTie diagram, Fishbone (or Ishikawa diagram) or display called "causal mapping."
They both have the same end point in that they are trying to find the root cause for a failure. One is a process or technique, the other is a display of the process or the results of the process.
Hope that helps