I'd say you have a gnat there.
The adults should not be harmful, but the larva might be breeding in decaying matter or organic fertilizer on the surface of your tree.
[img:d884e309e4]http://www.lausd.k12.ca.us/Belmont_HS/tkm/Pics/picket%20fence/Gnat/gnat.jpg
[/img:d884e309e4]
They also seem to like to hang around freshly watered moss. Might be good for a drink.
Regards
Gnatt
I mean Matt.