I have a lot of what I think is Clover in my garden. Should I be concerned?
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The picture on the left looks like clover - or most of it does. Perhaps wait until it has more leaves if you are unsure.
The picture on the right is very difficult to say. Most seedlings put out a very generic dicot pair of leaves - this is what the picture shows. True leaves will be next and will help with identification.
To me, clover is one of the benign weeds. Yes I pull it out of pots and beds, but usually let it be when it is in the lawn. It isn't spikey, tends not to drown things out (too much), and it is a legume - so its presence will add nitrogen to your soil.
Here in North Texas it can grow into big "balls" on lawns. Then, yes, it does look untidy. If it is tall enough, the best approach is a lawn mower, but you have to get it when it is tall enough that the mower decapitates all the leaves. That will keep it in check for at least another year. Often clover will stay low below that level. I also find it responds to a broad leaf lawn weedkiller - it doesn't need much, so a light spot treatment is fine.
The photo on the left does look like clover, but it could also be wood sorrel (oxalis). You'll know for sure once it flowers. Neither one should concern you, really. Although some people consider them to be unsightly in their lawns, clovers provide nectar for various insect species (especially bees) and some can be quite pretty. Some wood sorrels are considered ornamental so you may want to keep it if it turns out to be oxalis. It looks like it's in a location where it can easily be removed by hand so if you don't want the plant there just dig it up.
As winwaed writes, the seedlings in the right-hand photo could be almost anything.
The photo on the left looks like clover. The photo on the right is hard to tell; but if the soil came from the same place it seems likely that they are also clover.
Other answers have said that clover is mostly benign, but this depends: (a) on where you have them and (b) on what species of clover it is.
In my opinion, white clover makes a fine lawn (parts of my lawn are almost entirely white clover).
In the vegetable or flower garden, they can be a real nuisance. I used a batch of composted horse manure that was loaded with clover seed. Seedlings like what your right picture shows were growing in such thick clumps that it would have choked out anything I tried to direct-seed. Some types have deep roots (e.g. crimson clover has a taproot that goes 30cm). Some species have stolons or rhizomes. In my experience, pulling out mature clover with established roots next to seedlings or tender plants can damage the neighboring plant.