Welcome to the forum. Many good comments above, but my two cents.
No, you can't "over ferment" unless you were thinking of stopping for a sweet wine. However, as fermentation slows, the CO2 protecting the wine gets thin and you leave yourself open to bad bugs. Also, as noted above, the gross lees can be an issue unless you go for sur lees, which is a whole different thing. So, racking and topping off somewhere around .996 to 1 or a bit more is OK. As it is a white, erring on the "too soon" side would be my instinct. However, you don't have to panic as long as bubbles still come, just the risk increases.
I expect the white froth is the foam created by the fermentation. As it slows you need less headspace for the foam, and thus can rack it to a smaller vessel with less air, like very little. Put the airlock back on as it will continue for a while longer.
Good luck and enjoy.
Columbia Valley Washington