Can I use Plaxcaster Frogling to temporarily shut off Species Gorger's ability?
Species Gorger's triggered ability does not target. Abilities that target use the word "target". As such, it does not interact with Plaxcaster Frogling's activated ability or any other shroud effects.

In addition, because the triggered ability does not target, you make the choice of which creature to return while the ability is resolving (which means the choice can't be responded to in any way).