Blue plus

https://musescore.org/en/handbook/3/measure-operations#duration
(anacrusi)
See: https://musescore.org/en/node/284033

Measure is longer than time signature, actual duration longer than nominal duration.
Not sure why though, looks like 4/4, we'd need the score to check. Or is the previous measure resp. active time sig not 4/4?

For a bit of explanation. The blue plus tells you the measure is longer than the time signature. You have done something to add extra beats in those measures. If this was not intentional, then you can adjust the actual duration of the measure to match the Nominal duration as in Shoichi's link. Note that some of the notes in the measures you show will be deleted if you do this.