Yeah I would suggest its probably a crack...........caused by the crater solidifying too quickly. Try increasing your downslope and also remember to add another dab of tig wire just before the arc goes out.......also ensure you got plenty of post gas.
What type of joint and prep are you using, if its a line down the middle of the bead it sounds like hot cracking, or solidification cracking, caused as matt says, by the middle of the weld being the last part to solidify. aluminium has a high coefficient of expansion, the parent material is on the move while your welding it so you may need to allow for this in the set up.
Thats what I was getting at, its probably a scratch caused by the (very) hard ceramic on the soft aluminium, try holding off, you don't need to walk the cup to lay a bead on flat plate.
Why weave?, what thickness are you welding, aluminium weld pools tend to be wider than mild or stainless anyway, unless your on heavy stuff, if you hang about weaving, you'll end up blowing it away.
Good advice from the other posters, you state it is thin sheet, then stringer run is more than suffice, forget weaving, you are wasting time and energy. To your original Q, sounds to me like crater cracks, others have mentioned how to eliminate this.
If you are welding say 1-1.6 mm ally use a 2.4 filler, gets you Bootiful scales, all nice and shiny, and you don't need to feed as much as a 1.6 rod and less chance of blowing through too.........