Can't tell you with any degree of certainty :-) Far too many cultivars with very similar leaf shape and color - even the noted JM authorities have difficulty in ID'ing them accurately without any unusual characteristic to differentiate. Is it grafted or a seedling?

It looks like the Japanese Maple 'Ryusen'.

Grafted

Let me back up on it being grafted. I need to double check again.

Possibly 'Ryusen'. Possibly 'Jiro Shidare'. Or possibly just another bright green palmate leafed JM with a lot of recent growth that is producing a somewhat cascading appearance. The habit really doesn't have the very pronounced weeping of the two former possibilities.