I think these are structures from the Southern Dinaric Karst System, the main geological structure and something quite common in that area. Karst comes from the croatian word <i>krš</i> and means stony dour soil. Most of your objects seems to be calcitic. The original sediments of the area, mostly limestone, was dissolved and finaly precipitated in structures like the ones you presented us here. So No, these are not fossil but Yes, these structures are quite interessting. Btw: That mountain is nice for skiing and was host for the the alpine competitions of the Olympic Winter Games of 1984. Very nice area!!
Hi Amina @amina-gojak, I agree with Ronny (@rleder), these appear to carbonate rocks/minerals. We saw some similar structures in Panama. Here is a picture of a limestone ‘tube’ from Lago Alajuela that is similar to the first image in your second set of photos. As Ronny explained, the carbonate dissolves and re-precipitates in ways that give the appearance of something once living, such as a burrow, but are in fact inorganic. Definitely still fun to find though!