Compared to the other infi-cast spells provided by the GSF feats, Melf's Acid Arrow sticks out as perhaps the worst one of them all:
It does not scale well, providing at best a pathetic average of 10.5 damage, easily beaten by Acid Splash (a cantrip) at twelve wizard levels when it goes up to 4d6.
The acid damage type is very commonly resisted, as compared to something like the force damage of Magic Missile. More often than not, your poor acid arrow will do 0 damage.
It has a "special" ability of adding a whole 1d6 of damage every round until it expires, but more often than not you'll expire first after the orc you fired it at is now chasing angrily after you.
Some of the other GSF spells have great utility and are useful at all levels, like See Invisibility or Endure Elements. Burning Hands and Color Spray both provide a low-level wizard with a much-needed mercy of being able to kill weaker monsters and get a fighting chance at completing early writ work, as it's hard enough already to be a wizard. Melf's Acid Arrow is terrible at all levels.
As a replacement, there are a few other level 1 and 2 arcane conjuration spells that could serve as a viable alternative. Mage Armor is the most obvious one, as the +4 AC would increase survivability early on until equipment starts to provide AC. Grease and Web are fairly weak, but would still provide some utility in trapping monsters. I'm sure there are some others that might exist that I'm missing, but you get the point.
Melf's Acid Arrow sucks.