items, blocks and the like get a weird trail effect and noise effect

(noise around the potion)



(trail thing on hand)

(a little noise should be seen but might not be due to low quality(sorry))

note: this isn't only in caves. the cave images only happen around time 4000 - 12000

The noise is the result of sampling, it can be from the bloom or some other effect. It is notorious because the number is kept low to have speed.

The trail is the result of temporal supersampling that makes antialiasing possible (and also makes that noise you say less noticeable).

It is the normal operation of the shader.