The most common and most likely answer to this crossword clue is the 7 letter word NOSTOCS
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The straight part
We know that "algae" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case algae = nostocs.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
Crossword Answer definitions
NOSTOCS noun- Any member of the genus Nostoc of cyanobacteria, found in a variety of environmental niches, that form colonies composed of filaments of moniliform cells in a gelatinous sheath.