Help Gardening gurus - slime under planters - what is it and how can I get rid of it?

[dinkystinky]

Have been scouring the internet for advice but cant find anything on point so wondered if the gardening gurus of MN can help.

I have 4 planters on a wall outside our house - they have holes at the bottom for drainage and have small box hedges planted in them. A couple of months ago MIL came and did some super soaking of the plants in the boxes as she said they were too dry (she's an avid gardener, I'm not). A few days afterwards, lots of brown slime (which if left goes abit greeny) started coming out from under one of the planters. It appears in abundance any time its watered/it rains and is impossible to get off the wall with soap and water. The other 3 planters are fine. Any idea what it is and what I can do to stop it occurring again? I havent watered the plants in a few days but its still coming out from under the planters and is horrible. Should I remove the plants and start afresh with new box hedges?

How big are the planters, and how recently have they been repotted with fresh compost? Are the box hedges healthy looking and growing well? Any noticeable difference between the plants in the one which slimes and those in the three which don't?

planters about 70 cm long - not recently repotted (MIL simply drenched the plants). Box hedges growing ok (new growth) but not as well as others on the wall - the bottom of the ones in the slimy one do look quite dry though (probably because I've been scared to water them!) Any idea what it might be?