There two types of how cameras report motion: Some just send a “motion detected” event, but others send two events: “motion detected” and “no more motion detected”. It looks like the Arlo cam is of the former type: https://ifttt.com/arlo/details
So you will need a Boolean variable (e.g. name it “motion_detected”), a condition with a timeout (How to setup time constraints for Conditions: delays and timeouts) and one Logicblock.
I will try to find an example in the community that is similar to your case to have more details.
One more hint: IFTTT has changed the plans, and the free plan includes now 5 applets of any kind (template based or custom): https://ifttt.com/plans