IMO, Know set rules on how thet act at night but it is normal for them to retract into the rocks some or all at night...its a defence from predation..its why they like haveing their foot deep in the rocks, so they can get away from preditors...IMO, after a while they may/should do it less and less...especally if they have clowns hosting/protecting them...
Mine rarely retracts very much any more but some times it will, like maybe once every 2 weeks or so...but when it does, the Clowns just hang around it