Lilac infected?

Hi all, hoping someone can help! We have a lilac in our garden when we moved in last March and are quite new to gardening. Last year and this year it flowered and has grown well with healthy looking leaves. However, we've recently noticed what looks like some kind of infection at the base of the lilac. Does anyone know what this is/if it is harmful/how it can be treated if so? Attached some pictures (please ignore the lack of pruning and removing of dead flowers, we've just come back from holiday!). Thanks