What is the meaning of 'acala sukha'? What is the exact meaning of 'acala sukha'? Is it mentioned in the Pali-canon? If so, where ? In what context ?
The PTS dictionary says,


  Cala (adj.) [see calati] moving, quivering; unsteady, fickle, transient S iv.68 (dhammā calā c' eva vyayā ca aniccā, etc.); J ii.299; iii.381; v.345; Miln 93, 418; Sdhp 430, 494.
  -- acala steadfast, immovable S i.232; J i.71 (ṭṭhāna); Vv 514 (˚ṭṭhāna=Ep. of Nibbāna); acalaŋ sukhaŋ (=Nibbāna) Th 2, 350; cp. niccala motionless DhA iii.38


So cala means "unsteady", acala means "not unsteady", and acala sukha means "unwavering happiness" i.e. nibbana.

The phrase is in some of the suttas, e.g. the last line of Ud 8.10; and the end of the first verse of Thag 3.16.