[chaghalibaghali]

Interestingly something similar (but chemically different) happened in Australia recently too: https://www.standard.co.uk/news/world/bondi-beach-tar-balls-...

[askvictor]

These turned out to be fatbergs (i.e. sewerage that hadn't broken down due to people flushing so-called 'flushable' wipes down the toilet)