No errata and little hard rules: my interpretations of similar powers and the existing rules that may affect it.
First, let's define the power:
Misshapen Utility 6 Dread Carapace
Effect: Until the end of the encounter, you gain a +2 power bonus to AC and Fortitude. In addition, when any enemy ends its turn adjacent to you, it must use a free action to move up to its speed away from you.
Published in Dragon Magazine 416.
Second let's define a move:
Move actions involve movement from one place to another.
- Crawl - While prone, move up to half speed
- Escape - Escape a grab and shift 1 square
- Run - Move up to speed + 2; grant combat advantage until next turn and take a -5 penalty to attack rolls
- Stand up - Stand up from prone
- Shift - Move 1 square without provoking opportunity attacks
- Squeeze - Reduce size by one category, move up to half speed, and grant combat advantage
- Walk - Move up to walking speed
How it works out
1. Since the power says they must move away, moving zero squares as part of the free action is not valid. Likewise while standing up or going prone are move actions, they do not move the monster any further away and are also invalid for the free action move.
2. Shifting is a move action and it moves the creature away from you. Shifting is valid.
3. IF the creature has a defined teleport speed, they may teleport, however they can not teleport as part of a power.
4. Crawling is a valid move. In the example you give the monster could crawl up to 4 squares away. Crawling is defined as 1/2 your move speed. There is a feat that lets you shift while prone (Low Crawl) but that is only available to PCs, not monsters.