Well Ricki does seem to play safe with the more experienced players rather than risk playing a youth player. Nothing wrong with that, but he's weakness is to provide the match setting environment for the youth player to improve on. Sadly, training matches and practices are a poor second for the actual league match for experience. You learn more playing against other players you don't know about and adapt accordingly. Playing against the same set of expreienced players in the Nix practices might give you a large amount of information on how to play better ( based on their experience and not your own) but you have to develop it and work on your own game on the field in an actual do or die match. Many youth players will have the newbie experience in a professional club but they have to get out of that stage and start applying it in a real situtation.
With Musa, people may think that his weakness is his lack of speed/agility but he makes up for it with better positional play and similar to what happen Kosta and even Roja to some extent, he was simply not given enough A-League on field experience. However having said that, without a youth team to build up his newly acquired football knowledge, like any other youth, he will lose the needed playing time to focus on his game and will likely to suffer staying in that environment. The games in the NZFC was good for him, but you hardly able to harden him for A-League level since the quality is a huge step down and the training is different to what he needs.
Anyway, you have to remember Musa is a young player still and defenders tends to mature much later than the rest of the other team positions at that level because they are on the back foot with a simple mistake being punished by letting the opposition in with a goal. I still think he would mature better in a holding central midfielder position rather than at CB where there is less demand on his agility and turn and optimise his passing and positioning game that he is more known for and develop more combative approach that would help his aggressiveness in defence and urgency in setting up an attack from a central position.
Fulham is a great environment for him, and plenty of viable options as a football player. I hope he will extend his stay and it will lead on to bigger and better things for him.