From what I can make out from the pictures provided, I think your identification  as Chroococcidium is correct. Please search for the following features : Unicellular - colonial- in small agglomerations , irregularly localized in fine, diffluent, colourless, sometimes slightly lamellated mucilaginous envelopes; colonies microscopic, but sometimes form macroscopically visible gelatinous masses. Cells spherical, hemispherical, irregular spherical, oval, more or less rounded, usually of different size in one colony, rarely irregular, blue-green; usually without own mucilaginous envelopes. source:  http://www.cyanodb.cz/Chroococcidium.

Good luck!

Certainly the picture quality is not good enough to identify any unicellular cyanobacteria for sure. Moreover, most unicellular cyanobacteria thriving on rocks may have stress morphology, which needs to be revived or multiple environmental samples may resolve identification. I would expect new images of better quality before proper identification. If you have enough sample, we may help identifying the same using standard taxonomic publications that we use routinely.

Thanks

Sushanta