Is it over the whole lawn or the odd bit here and there?

When I found the same sort of thing on my lawn, not too much of it though, I decided, rightly or wrongly, that the little birds who do my aerating for me had probably introduced a bit of rogue grass seed from their travels. So I removed the clumps as best I could without damaging the lawn too much, then re seeded if it needed.

I cut the lawns of 36 customers. Your problem sometimes occurs on their lawns.

As Scrumpy says seeds of different grasses do find their way in either on the wind ,on your shoes or via birds/other animals.

Unfortunately the usual weed killers watered/sprayed on won't work because they would kill the grass you want to keep as well as the grass you don't .

There are no easy options but if you only have a few patches of this "new" grass these methods work but require patience and care.

Allow the grass you want rid of to grow longer( mow around it for a week or two) Then carefully paint the stalks of these grasses with weed killer. After a few days you can return to mowing and the grass you want gone will be dead

Alternatively you could dig up the patches of unwanted grass and put new seed down in its place.

If you have unwanted grass all over the lawn you will either have to just put up with it or weedkill the whole lawn and reseed or returf