Hallo, one of the options is that your structures could belong to pycnidia of a lichen. Are any algae associated? To my mind  'Biatora' albohyalina comes which is frequently present only in pycnidial state of pallid half-immersed pycnidia. Also some members of the genus Micarea may occur only in anamorphic stage (small 'micareoid algae' of 4-7(8) µm should be present); long-macroconidial taxa of Micarea also often contain gyrophoric acid in the wall of pycnidia and/or in thallus (or in apothecia when fertile), so they should react C+ fleetingly reddish by Calcium-chloride. There likely exist more lichens with similar conidia but they seem to be slimmer than 3 µm. At least 'Biatora' albohyalina and Micarea taxa should have thinner macroconidia. This is just for your consideration, who knows what it is? :-).

Zdenek