This is an Aphid Gall on Wych Elm (Ulmus glabra) leaves.
As with all galls it is important to get sight of the 'culprit'. In the case of Eriosoma ulmi it is essential because it is not the only Galler of Wych elm leaves.
It is rare to see the Aphid outside the leaf roll but, when seen, it will need to be confirmed as a wax-covered fundatrix. These are found inside the roll. The home is often a sweet sticky mess but it is usually easy to find several fundatrices.
There are at least 75 Elm Aphids in the World
.
For good ID confirmation, a photo of the aphids is usually necessary
.
https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/hemiptera/sternorrhyncha/aphidoidea/aphididae/eriosomatinae/eriosomatini/eriosoma/eriosoma-ulmi/
and
https://influentialpoints.com/Gallery/Eriosoma_ulmi_Elm-currant_aphid.htm