Encrusting organism found on the underside of a submerged branch. My initial thought was a sponge, based on form and texture however the structure shown in image 2 doesn't look sponge-like.
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025 Freshwater Sponge Spongillidae
1999/07/07 Kaurutootsi, Valgamaa, Estland
57.9760500 26.48649
1999/07/07 .https://photos.app.goo.gl/3jkBpYP15sAGYphb9
https://www.google.com/maps/place/57%C2%B058'33.6%22N+26%C2%B029'20.4%22E/@57.9711425,26.5002587,3a,75y,298.7h,75.41t/data=!3m7!1e1!3m5!1sqULi0J6r-io11GA0Bn5wLA!2e0!6s%2F%2Fgeo2.ggpht.com%2Fcbk%3Fpanoid%3DqULi0J6r-io11GA0Bn5wLA%26output%3Dthumbnail%26cb_client%3Dsearch.gws-prod.gps%26thumb%3D2%26w%3D211%26h%3D120%26yaw%3D28.760406%26pitch%3D0%26thumbfov%3D100!7i13312!8i6656!4m5!3m4!1s0x0:0x0!8m2!3d57.976!4d26.489
1999/07/07 .https://photos.app.goo.gl/3jkBpYP15sAGYphb9
1999/07/07 Kaurutootsi, Valgamaa, Estland
57.9760500 26.48649
1999/07/07 .https://photos.app.goo.gl/3jkBpYP15sAGYphb9
https://www.google.com/maps/place/57%C2%B058'33.6%22N+26%C2%B029'20.4%22E/@57.9711425,26.5002587,3a,75y,298.7h,75.41t/data=!3m7!1e1!3m5!1sqULi0J6r-io11GA0Bn5wLA!2e0!6s%2F%2Fgeo2.ggpht.com%2Fcbk%3Fpanoid%3DqULi0J6r-io11GA0Bn5wLA%26output%3Dthumbnail%26cb_client%3Dsearch.gws-prod.gps%26thumb%3D2%26w%3D211%26h%3D120%26yaw%3D28.760406%26pitch%3D0%26thumbfov%3D100!7i13312!8i6656!4m5!3m4!1s0x0:0x0!8m2!3d57.976!4d26.489
1999/07/07 .https://photos.app.goo.gl/3jkBpYP15sAGYphb9
I never found a fresh water sponge, but maybe I did today? Firm, not slimy.
There were several of these masses, growing just ABOVE the waterline. The level of the lake seems stable. Could these be Freshwater Sponges with the algae?