there should be a shiny substance along edges, "glistening dew" ...
The top leaves are clear, so that's unusual as pests do not care where on the plant they chew. And, as long as not the start of leaf miners, it's fine.
Keep an aye on it. if isolated to bottom leaf and does not spread, it's just an ugle bottom leaf. Inspect plants for bugs. Use a magnifying glass because some are at the limits of the human eye from a 1 foot viewing distance.
if you find somethign cralwing on them, go buy some "pyrethrine" based product or similar.. i'd suggest something concentrated you mix together in your own spray bottle.