The most common and most likely answer to this crossword clue is the 6 letter word DIATOM
D
I
A
T
O
M
Buddy explains!
The straight part
We know that "alga" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case alga = diatom.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.
What does DIATOM mean?
DIATOM noun- Any of a group of minute unicellular algae having a siliceous covering of great delicacy, now categorized as class Diatomophyceae or division Bacillariophyta.