der Fleck
stain
A small dirty or discolored area on a surface.
Ich habe einen Kaffeefleck auf meinem neuen Hemd.
I have a coffee stain on my new shirt.

stain / spot
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stain
A small dirty or discolored area on a surface.
Ich habe einen Kaffeefleck auf meinem neuen Hemd.
I have a coffee stain on my new shirt.

The two main meanings can be distinguished well from context. A 'Fleck' (stain) on clothing or surfaces is usually unwanted dirt or discoloration. A 'Fleck' (spot) in a landscape, on the other hand, is a place described neutrally or positively.
Ich habe einen Grasfleck auf meiner neuen Hose.
I have a grass stain on my new pants.
When referring to dirt or staining, 'Fleck' (stain) is often combined with verbs like 'machen' (to make), 'entfernen' (to remove), or 'rausbekommen' (to get out). It is also part of many compounds such as 'Kaffeefleck' (coffee stain), 'Grasfleck' (grass stain), or 'Fettfleck' (grease stain).
Wie bekomme ich diesen hartnäckigen Rotweinfleck aus dem Teppich?
How do I get this stubborn red wine stain out of the carpet?
When a place is meant, 'Fleck' (spot) is often used with descriptive adjectives like 'schön' (beautiful), 'ruhig' (quiet), or 'idyllisch' (idyllic). A common phrase is also 'ein schöner Fleck Erde' (a beautiful spot on earth).
Für unser Picknick suchen wir uns einen sonnigen Fleck im Park.
For our picnic, we'll look for a sunny spot in the park.
The idiom 'sich nicht vom Fleck bewegen' or 'nicht vom Fleck kommen' is reflexive. It means that you are making no progress or are literally stuck in one place.
Wegen des vielen Schnees kam das Auto nicht mehr vom Fleck.
Because of all the snow, the car couldn't move anymore.
time / mark
'das Mal' and 'der Fleck' can both describe a visible spot on a surface or on the skin. 'das Mal' especially means a natural mark on the skin, for example a birthmark or mole. 'der Fleck' is more general and is often a discolored or dirty spot, for example on clothing, a table, or also on fur and skin. They can be confused because both look like a small spot. If the spot is a natural part of the body, 'Mal' often fits. If it comes from dirt, paint, or discoloration, 'Fleck' is more natural.

In der Küche entfernt Maria den Fleck vom Tisch, bevor die Gäste kommen.
In the kitchen, Maria removes the stain from the table before the guests arrive.
David hat ein kleines Mal am Hals, das schon seit seiner Kindheit da ist.
David has a small mark on his neck that has been there since childhood.