Blueberry-- Annoying as they are, fungus gnats themselves will not harm your plants. They seem to be attracted to moist soil rich in compost. It is difficult to help you with diagnosing the problem you're having as you didn't mention what type of plant/succulent you're dealing with. Possible to post a pic of the entire plant and a second pic zoomed into the problem area as well as the soil it's growing in. --Best

The presence of fungus gnats near container plants is an indication that the potting medium may be too moist. But, Neil, female fungus gnats look for moist peaty potting soil into which they will deposit their eggs.

It's the tiny worm-like larvae that feed on organic matter bits....in containers, this will include tender root tips and root hairs, which are vital to a plant's existence. The larvae aren't a big issue for plants growing in the ground, but very much are a problem for container grown plants.

Blueberry, please provide the pictures suggested by Neil, it will be very helpful.

Apologies Blueberry Rhizo_1 is correct about the larvae. I should have included that in my comment. When i switched my growing medium to a gritty mix the fungus gnats vanished. That was many years ago and maybe something has changed. Hopefully others will also comment soon and collectively you will get the best help for your situation. ---Best