Thanks for reporting this. I will have to take a closer look at the EtherCard library to see what may have caused this. The OpenSprinkler firmware relies on the open-source EtherCard library, and this ICMP request may be related.
It is a heartbeat ping to check connectivity to the router. Honestly it is not needed, but totally harmless. Unless you impose serious restrictions to device traffic this should never cause an issue. Other devices like the nest thermostat will do the same thing to establish and maintain a certain IP address. Essentially it never let’s the connection go stale resulting in a new DHCP assignment.