The rulebook FAQ says a spellcaster cannot cast Celestial shield on his own unit as he has no line of sight of it, being in the front row. I think you know what question is coming, but I will state it anyway; Can the Slaan see his own unit of TG?
Very interesting question, technically he is not right behind them since when he casts spells he raises up. I think the rule is intended that you can't cast on your own unit so he cannot, but it is definitely up for debate in a 'rules as written' way.
Since the Slann is in the second row, you DO have line of sight to your own unit. At all times. Next the Slann has line of sight equivalent to a Large Target when casting spells. This would allow him to see his own Temple Guard as well as anything else that isnt blocked by terrain. So yes, Celestial Shield should be 100% acceptable for a Slann on his own Temple Guard.