What it means is traffic is being received by that MX on a VLAN with IP addressing belonging to another VLAN.
For example, lets say you have configure this on your MX:
VLAN1: 192.168.1.1/24
VLAN2: 10.0.0.1/24
The MX should not receive traffic from 10.0.0.0/24 on VLAN1 - that traffic should be on VLAN2.