Also… If it isn’t condensation or whatever… are you sure it isn’t a weep on the compression joint then dripping down the pipe making it look like the white hep elbow?
Starting at highest point, put kitchen roll round the joint and if dry work downwards until you find where its is wet…
Dry it and check it is not condensation. Leave for a while and see if it is still wet.
It is the stop valve? There doesn’t appear to be any ptfe tape on it! If it is weeping, tighten slightly with an adjustable spanner. Not too much or it WILL leak.
The plastic connectors are push fit but the collar normally turns to lock the connection. Check it is tight. The connector will normally swivel round the pipe.
Disregard most of this post. The plastic fitting is a hep20 fitting, they are not designed to tighten up and as a result won’t. He is mistaking that for Speedfit.
The bit about the toilet paper and not ovvertightening is bang-on tho.